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Copyleft is a play
Word play
Word play or wordplay is a literary technique in which the words that are used become the main subject of the work, primarily for the purpose of intended effect or amusement...

 on the word copyright
Copyright
Copyright is a legal concept, enacted by most governments, giving the creator of an original work exclusive rights to it, usually for a limited time...

to describe the practice of using copyright law to offer the right to distribute copies and modified versions of a work and requiring that the same rights be preserved in modified versions of the work. In other words, copyleft is a general method for making a program (or other work) free (libre
Gratis versus Libre
Gratis versus libre is the distinction between two meanings of the English adjective "free"; namely, "for zero price" and "with little or no restriction"...

), and requiring all modified and extended versions of the program to be free as well.

Copyleft is a form of licensing
License
The verb license or grant licence means to give permission. The noun license or licence refers to that permission as well as to the document recording that permission.A license may be granted by a party to another party as an element of an agreement...

 and can be used to maintain copyright conditions for works such as computer software
Computer software
Computer software, or just software, is a collection of computer programs and related data that provide the instructions for telling a computer what to do and how to do it....

, documents and art
Art
Art is the product or process of deliberately arranging items in a way that influences and affects one or more of the senses, emotions, and intellect....

. In general, copyright law is used by an author to prohibit others from reproducing, adapting, or distributing copies of the author's work. In contrast, under copyleft, an author may give every person who receives a copy of a work permission to reproduce, adapt
Derivative work
In United States copyright law, a derivative work is an expressive creation that includes major, copyright-protected elements of an original, previously created first work .-Definition:...

 or distribute it and require that any resulting copies or adaptations are also bound by the same licensing agreement.

Copyleft licenses (for software) require that information necessary for reproducing and modifying the work must be made available to recipients of the executable
Executable
In computing, an executable file causes a computer "to perform indicated tasks according to encoded instructions," as opposed to a data file that must be parsed by a program to be meaningful. These instructions are traditionally machine code instructions for a physical CPU...

. The source code
Source code
In computer science, source code is text written using the format and syntax of the programming language that it is being written in. Such a language is specially designed to facilitate the work of computer programmers, who specify the actions to be performed by a computer mostly by writing source...

 files will usually contain a copy of the license terms and acknowledge the author(s).

Copyleft type licenses are a novel use of existing copyright law to ensure a work remains freely available. The GNU General Public License
GNU General Public License
The GNU General Public License is the most widely used free software license, originally written by Richard Stallman for the GNU Project....

, originally written by Richard Stallman
Richard Stallman
Richard Matthew Stallman , often shortened to rms,"'Richard Stallman' is just my mundane name; you can call me 'rms'"|last= Stallman|first= Richard|date= N.D.|work=Richard Stallman's homepage...

, was the first copyleft license to see extensive use, and continues to dominate the licensing of copylefted software. Creative Commons
Creative Commons
Creative Commons is a non-profit organization headquartered in Mountain View, California, United States devoted to expanding the range of creative works available for others to build upon legally and to share. The organization has released several copyright-licenses known as Creative Commons...

, a non-profit organization
Non-profit organization
Nonprofit organization is neither a legal nor technical definition but generally refers to an organization that uses surplus revenues to achieve its goals, rather than distributing them as profit or dividends...

 founded by Lawrence Lessig
Lawrence Lessig
Lawrence "Larry" Lessig is an American academic and political activist. He is best known as a proponent of reduced legal restrictions on copyright, trademark, and radio frequency spectrum, particularly in technology applications, and he has called for state-based activism to promote substantive...

, provides a similar license called ShareAlike.

Reciprocity


Copyleft can be characterized as a copyright licensing scheme in which an author surrenders some but not all rights under copyright law. Instead of allowing a work to fall completely into the public domain
Public domain
Works are in the public domain if the intellectual property rights have expired, if the intellectual property rights are forfeited, or if they are not covered by intellectual property rights at all...

 (where no ownership of copyright is claimed), copyleft allows an author to impose some restrictions on those who want to engage in activities that would more usually be reserved by the copyright holder. Under copyleft, derived works may be produced provided they are released under the compatible copyleft scheme.

The underlying principle is that one benefits freely from the work of others but any modifications one makes must be released under compatible terms. For this reason some copyleft licenses are also known as reciprocal
Reciprocity (social psychology)
Reciprocity in social psychology refers to responding to a positive action with another positive action, rewarding kind actions. People categorize an action as kind by viewing its consequences and also by the person's fundamental intentions. Even if the consequences are the same, underlying...

 licenses, they have also been described as "viral" due to their self-perpetuating terms.
Under fair use
Fair use
Fair use is a limitation and exception to the exclusive right granted by copyright law to the author of a creative work. In United States copyright law, fair use is a doctrine that permits limited use of copyrighted material without acquiring permission from the rights holders...

, however, the copyleft license may be superseded, just like regular copyrights. Therefore, any person utilizing a copyleft-licensed source for their own work is free to choose any other license provided they meet the fair use standard.

While copyright law gives software authors control over copying, distribution and modification of their works, the goal of copyleft is to give all users of the software the freedom
Gratis versus Libre
Gratis versus libre is the distinction between two meanings of the English adjective "free"; namely, "for zero price" and "with little or no restriction"...

 to carry out these activities. In this way, copyleft licenses are distinct from other types of free software licenses, which do not guarantee that all "downstream" recipients of the program receive these rights, or the source code
Source code
In computer science, source code is text written using the format and syntax of the programming language that it is being written in. Such a language is specially designed to facilitate the work of computer programmers, who specify the actions to be performed by a computer mostly by writing source...

 needed to make them effective. In particular, permissive
Permissive free software licence
A permissive free software licence is a class of free software licence with minimal requirements about how the software can be redistributed. This is in contrast to copyleft licences, which have reciprocity / share-alike requirements. Both sets of free software licences offer the same freedoms in...

 free software licenses such as BSD
BSD licenses
BSD licenses are a family of permissive free software licenses. The original license was used for the Berkeley Software Distribution , a Unix-like operating system after which it is named....

 allow re-distributors to remove some or all these rights, and do not require the distribution of source code.

History



An early example of copyleft was the Tiny BASIC
Tiny BASIC
Tiny BASIC is a dialect of the BASIC programming language that can fit into as little as 2 or 3 KB of memory. This small size made it invaluable in the early days of microcomputers , when typical memory size was only 4–8 KB.- History :...

 project started in the newsletter of the People's Computer Company in 1975. Dennis Allison wrote a specification for a simple version of the BASIC programming language. This design did not support text strings and only used integer arithmetic. The goal was for the program to fit in 2 to 3 kilobyte
Kilobyte
The kilobyte is a multiple of the unit byte for digital information. Although the prefix kilo- means 1000, the term kilobyte and symbol KB have historically been used to refer to either 1024 bytes or 1000 bytes, dependent upon context, in the fields of computer science and information...

s of memory.

The Tiny BASIC contents of the newsletter soon became Dr. Dobb's Journal
Dr. Dobb's Journal
Dr. Dobb's Journal was a monthly journal published in the United States by CMP Technology. It covered topics aimed at computer programmers. DDJ was the first regular periodical focused on microcomputer software, rather than hardware. It later became a monthly section within the periodical...

 of Tiny BASIC
with a subtitle of "Calisthenics & Orthodontia, Running Light Without Overbyte." Hobbyists began writing BASIC language interpreters for their microprocessor-based home computers and sending the source code to Dr. Dobb's Journal and other magazines to be published. By the middle of 1976, Tiny BASIC interpreters were available for the Intel 8080
Intel 8080
The Intel 8080 was the second 8-bit microprocessor designed and manufactured by Intel and was released in April 1974. It was an extended and enhanced variant of the earlier 8008 design, although without binary compatibility...

, the Motorola 6800
Motorola 6800
The 6800 was an 8-bit microprocessor designed and first manufactured by Motorola in 1974. The MC6800 microprocessor was part of the M6800 Microcomputer System that also included serial and parallel interface ICs, RAM, ROM and other support chips...

 and MOS Technology 6502
MOS Technology 6502
The MOS Technology 6502 is an 8-bit microprocessor that was designed by Chuck Peddle and Bill Mensch for MOS Technology in 1975. When it was introduced, it was the least expensive full-featured microprocessor on the market by a considerable margin, costing less than one-sixth the price of...

 processors. This was a free software project before the internet allowed easy transfer of files. Computer hobbyists would exchange paper tapes, cassettes or even retype the files from the printed listings.

Jim Warren, editor of Dr. Dobb's Journal, wrote in the July 1976 ACM
Association for Computing Machinery
The Association for Computing Machinery is a learned society for computing. It was founded in 1947 as the world's first scientific and educational computing society. Its membership is more than 92,000 as of 2009...

 Programming Language newsletter about the motivations and methods of this successful project. He started with this: "There is a viable alternative to the problems raised by Bill Gates in his irate letter to computer hobbyists
Open Letter to Hobbyists
The Open Letter to Hobbyists was an open letter written by Bill Gates, the co-founder of Microsoft, to early personal computer hobbyists, in which Gates expresses dismay at the rampant copyright infringement taking place in the hobbyist community, particularly with regard to his company's...

 concerning 'ripping off' software. When software is free, or so inexpensive that it's easier to pay for it than to duplicate it, then it won't be 'stolen'." The method was to have an experienced professional do the overall design and then outline an implementation strategy. Knowledgeable amateurs would implement the design for a variety of computer systems. Warren predicted this strategy would be continued and expanded.

The May 1976 issue of Dr. Dobbs Journal had Li-Chen Wang
Li-Chen Wang
Dr. Li-Chen Wang is an American computer engineer, best known for his Palo Alto Tiny BASIC for Intel 8080-based microcomputers.This was the fourth version of Tiny BASIC that appeared in Dr. Dobb's Journal of Computer Calisthenics & Orthodontia, but probably the most influential. It appeared in the...

's Palo Alto Tiny BASIC for the Intel 8080 microprocessor. The listing began with the usual title, author's name and date but it also had "@COPYLEFT ALL WRONGS RESERVED". A fellow Homebrew Computer Club
Homebrew Computer Club
The Homebrew Computer Club was an early computer hobbyist users' group in Silicon Valley, which met from March 5, 1975 to December 1986...

 member, Roger Rauskolb, modified and improved Li-Chen Wang's program and this was published in the December 1976 issue of Interface Age magazine. Roger added his name and preserved the COPYLEFT Notice.

A later instance of copyleft arose when Richard Stallman
Richard Stallman
Richard Matthew Stallman , often shortened to rms,"'Richard Stallman' is just my mundane name; you can call me 'rms'"|last= Stallman|first= Richard|date= N.D.|work=Richard Stallman's homepage...

 was working on a Lisp
Lisp programming language
Lisp is a family of computer programming languages with a long history and a distinctive, fully parenthesized syntax. Originally specified in 1958, Lisp is the second-oldest high-level programming language in widespread use today; only Fortran is older...

 interpreter. Symbolics
Symbolics
Symbolics refers to two companies: now-defunct computer manufacturer Symbolics, Inc., and a privately held company that acquired the assets of the former company and continues to sell and maintain the Open Genera Lisp system and the Macsyma computer algebra system.The symbolics.com domain was...

 asked to use the Lisp interpreter, and Stallman agreed to supply them with a public domain version of his work. Symbolics extended and improved the Lisp interpreter, but when Stallman wanted access to the improvements that Symbolics had made to his interpreter, Symbolics refused. Stallman then, in 1984, proceeded to work towards eradicating this emerging behavior and culture of proprietary software
Proprietary software
Proprietary software is computer software licensed under exclusive legal right of the copyright holder. The licensee is given the right to use the software under certain conditions, while restricted from other uses, such as modification, further distribution, or reverse engineering.Complementary...

, which he named software hoarding.

As Stallman deemed it impractical in the short term to eliminate current copyright law and the wrongs he perceived it perpetuated, he decided to work within the framework of existing law
Law
Law is a system of rules and guidelines which are enforced through social institutions to govern behavior, wherever possible. It shapes politics, economics and society in numerous ways and serves as a social mediator of relations between people. Contract law regulates everything from buying a bus...

; In 1988, he created his own copyright license, the Emacs General Public License, the first copyleft license. This later evolved into the GNU General Public License
GNU General Public License
The GNU General Public License is the most widely used free software license, originally written by Richard Stallman for the GNU Project....

, which is now one of the most popular Free Software licenses. For the first time a copyright holder had taken steps to ensure that the maximal number of rights be perpetually transferred to a program's users, no matter what subsequent revisions anyone made to the original program. This original GPL did not grant rights to the public at large, only those who had already received the program; but it was the best that could be done under existing law.

The new license was not at this time given the copyleft label. Richard Stallman stated that the use of "Copyleft" comes from Don Hopkins
Don Hopkins
Don Hopkins is an artist and programmer specializing in human computer interaction and computer graphics.He inspired Richard Stallman, who described him as a "very imaginative fellow", to use the term copyleft. He coined Deep Crack as the name of the EFF DES cracker, and built "AJAXian"...

, who mailed him a letter in 1984 or 1985 on which was written: "Copyleft—all rights reversed." The term "kopyleft" with the notation "All Rites Reversed
All Rites Reversed
All rights reversed is a phrase that indicates a release of copyright or a copyleft licensing status. It is a pun on the common copyright disclaimer "All rights reserved", a formality originally required by the Buenos Aires Convention of 1910...

" was also in use in the early 1970s within the Principia Discordia
Principia Discordia
Principia Discordia is a Discordian religious text written by Greg Hill and Kerry Thornley . It was originally published under the title "Principia Discordia or How The West Was Lost" in a limited edition of 5 copies in 1965...

, which may have inspired Hopkins or influenced other usage. And in the arts Ray Johnson
Ray Johnson
Raymond Edward Johnson , known primarily as a collagist and correspondence artist, was a seminal figure in the history of Neo-Dada and early Pop art...

 had earlier coined the term independently as it pertained to his making of and distribution of his mixed media imagery in his mail art
Mail art
Mail art is a worldwide cultural movement that began in the early 1960s and involves sending visual art through the international postal system. Mail Art is also known as Postal Art or Correspondence Art...

 and ephemeral gifts, for which he encouraged the making of derivative work
Derivative work
In United States copyright law, a derivative work is an expressive creation that includes major, copyright-protected elements of an original, previously created first work .-Definition:...

s. (While the phrase itself appears briefly as (or on) one of his pieces in the 2002 documentary How to Draw a Bunny
Ray Johnson
Raymond Edward Johnson , known primarily as a collagist and correspondence artist, was a seminal figure in the history of Neo-Dada and early Pop art...

, Johnson himself is not referenced in the 2001 documentary Revolution OS
Revolution OS
Revolution OS is a 2001 documentary film that traces the twenty-year history of GNU, Linux, open source, and the free software movement.Directed by J. T. S. Moore, the film features interviews with prominent hackers and entrepreneurs including Richard Stallman, Michael Tiemann, Linus Torvalds,...

.)

Some have suggested that copyleft became a divisive issue in the ideological strife between the Open Source Initiative
Open Source Initiative
The Open Source Initiative is an organization dedicated to promoting open source software.The organization was founded in February 1998, by Bruce Perens and Eric S. Raymond, prompted by Netscape Communications Corporation publishing the source code for its flagship Netscape Communicator product...

 and the free software movement
Free software movement
The free software movement is a social and political movement with the goal of ensuring software users' four basic freedoms: the freedom to run their software, to study and change their software, and to redistribute copies with or without changes. The alternative terms "software libre", "open...

. However, there is evidence that copyleft is both accepted and proposed by both parties:
  • Both the OSI and the FSF have copyleft and non-copyleft licenses in their respective lists of accepted licenses.
  • The OSI's original Legal Counsel Lawrence Rosen
    Lawrence Rosen
    Lawrence Rosen is an attorney and computer specialist. He is a founding partner of Rosenlaw & Einschlag, a Californian technology law firm, specializing in intellectual property protection, licensing and business transactions for technology companies...

     has written a copyleft license, the Open Software License
    Open Software License
    The Open Software License is a software license created by Lawrence Rosen. The Open Source Initiative has certified it as an open source license, but the Debian project judged version 1.1 to be incompatible with the DFSG...

    .
  • The OSI's licensing how-to recognises the GPL as a "best practice" license.
  • Some of the software programs of the GNU Project are published under non-copyleft licenses.
  • Stallman himself has endorsed the use of non-copyleft licenses in certain circumstances, most recently in the case of the Ogg Vorbis license change.

Applying copyleft


Common practice for using copyleft is to codify the copying terms for a work with a license
License
The verb license or grant licence means to give permission. The noun license or licence refers to that permission as well as to the document recording that permission.A license may be granted by a party to another party as an element of an agreement...

. Any such license typically gives each person possessing a copy of the work the same freedoms as the author, including (from the Free Software Definition):
Freedom 0 - the freedom to use the work,
Freedom 1 - the freedom to study the work,
Freedom 2 - the freedom to copy and share the work with others,
Freedom 3 - the freedom to modify the work, and the freedom to distribute modified and therefore derivative works.


(Note that the list begins from 0 due to a coding tradition — the first array element in C
C (programming language)
C is a general-purpose computer programming language developed between 1969 and 1973 by Dennis Ritchie at the Bell Telephone Laboratories for use with the Unix operating system....

 and many other programming languages is numbered as 0.)

These freedoms do not ensure that a derivative work
Derivative work
In United States copyright law, a derivative work is an expressive creation that includes major, copyright-protected elements of an original, previously created first work .-Definition:...

 will be distributed under the same liberal terms. In order for the work to be truly copyleft, the license has to ensure
Obligation
An obligation is a requirement to take some course of action, whether legal or moral. There are also obligations in other normative contexts, such as obligations of etiquette, social obligations, and possibly...

 that the author of a derived work can only distribute such works under the same or equivalent license.

In addition to restrictions on copying, copyleft licenses address other possible impediments. These include ensuring the rights cannot be later revoke
Revoke
To annul by withdrawing.In trick-taking card games, a revoke is a violation of important rules regarding the play of tricks serious enough to render the round invalid...

d and requiring the work and its derivatives to be provided in a form that facilitates modification. In software
Computer software
Computer software, or just software, is a collection of computer programs and related data that provide the instructions for telling a computer what to do and how to do it....

, this requires that the source code
Source code
In computer science, source code is text written using the format and syntax of the programming language that it is being written in. Such a language is specially designed to facilitate the work of computer programmers, who specify the actions to be performed by a computer mostly by writing source...

 of the derived work is made available together with the software itself.

Copyleft licenses necessarily make creative use of relevant rules and laws. For example, when using copyright law, those who contribute to a work under copyleft usually must gain, defer or assign copyright holder status. By submitting the copyright of their contributions under a copyleft license, they deliberately give up some of the rights that normally follow from copyright, including the right to be the unique distributor of copies of the work.

Some laws used for copyleft licenses vary from one country to another, and may also be granted in terms that vary from country to country. For example, in some countries it is acceptable to sell a software product without warranty, in standard GNU GPL style (see articles 11 and 12 of the GNU GPL version 2), while in most Europe
Europe
Europe is, by convention, one of the world's seven continents. Comprising the westernmost peninsula of Eurasia, Europe is generally 'divided' from Asia to its east by the watershed divides of the Ural and Caucasus Mountains, the Ural River, the Caspian and Black Seas, and the waterways connecting...

an countries it is not permitted for a software distributor to waive
Waiver
A waiver is the voluntary relinquishment or surrender of some known right or privilege.While a waiver is often in writing, sometimes a person's actions can act as a waiver. An example of a written waiver is a disclaimer, which becomes a waiver when accepted...

 all warranties regarding a sold product. For this reason the extent of such warranties are specified in most European copyleft licenses. Regarding that, see the European Union Public Licence EUPL, or the CeCILL license, a license that allows one to use GNU GPL (see article 5 of the EUPL and article 5.3.4 of CeCILL) in combination with a limited warranty (see article 7 and 8 of the EUPL and 9 of CeCILL).

Types of copyleft and relation to other licenses



Copyleft is a distinguishing feature of some free software
Free software
Free software, software libre or libre software is software that can be used, studied, and modified without restriction, and which can be copied and redistributed in modified or unmodified form either without restriction, or with restrictions that only ensure that further recipients can also do...

 licenses. Many free software licenses are not copyleft licenses because they do not require the licensee to distribute derivative works under the same license. There is an ongoing debate as to which class of license provides the greater degree of freedom. This debate hinges on complex issues such as the definition of freedom and whose freedoms are more important, or whether to maximize the freedom of all potential future recipients of a work (freedom from the creation of proprietary software). Non-copyleft free software licenses maximize the freedom of the initial recipient (freedom to create proprietary software).

In common with the Creative Commons
Creative Commons
Creative Commons is a non-profit organization headquartered in Mountain View, California, United States devoted to expanding the range of creative works available for others to build upon legally and to share. The organization has released several copyright-licenses known as Creative Commons...

 share-alike licensing system, GNU's Free Documentation License allows authors to apply limitations to certain sections of their work, exempting some parts of their creation from the full copyleft mechanism. In the case of the GFDL, these limitations include the use of invariant sections, which may not be altered by future editors. The initial intention of the GFDL was as a device for supporting the documentation
Documentation
Documentation is a term used in several different ways. Generally, documentation refers to the process of providing evidence.Modules of Documentation are Helpful...

 of copylefted software. However, the result is that it can be used for any kind of document.

Strong and weak copyleft


The strength of the copyleft governing a work is an expression of the extent that the copyleft provisions can be efficiently imposed on all kinds of derived works. "Weak copyleft" refers to licenses where not all derived works inherit the copyleft license; whether a derived work inherits or not often depends on the manner in which it was derived.

"Weak copyleft" licenses are generally used for the creation of software libraries, to allow other software to link to the library, and then be redistributed without the legal requirement for the work to be distributed under the library's copyleft license. Only changes to the weak copylefted software itself become subject to the copyleft provisions of such a license, not changes to the software that links to it. This allows programs of any license to be compiled and linked against copylefted libraries such as glibc
GNU C Library
The GNU C Library, commonly known as glibc, is the C standard library released by the GNU Project. Originally written by the Free Software Foundation for the GNU operating system, the library's development has been overseen by a committee since 2001, with Ulrich Drepper from Red Hat as the lead...

 (the GNU project
GNU Project
The GNU Project is a free software, mass collaboration project, announced on September 27, 1983, by Richard Stallman at MIT. It initiated GNU operating system development in January, 1984...

's implementation of the C standard library
C standard library
The C Standard Library is the standard library for the programming language C, as specified in the ANSI C standard.. It was developed at the same time as the C POSIX library, which is basically a superset of it...

), and then redistributed without any re-licensing required.

The most well known free software license that uses strong copyleft is the GNU General Public License. Free software licenses that use "weak" copyleft include the GNU Lesser General Public License
GNU Lesser General Public License
The GNU Lesser General Public License or LGPL is a free software license published by the Free Software Foundation . It was designed as a compromise between the strong-copyleft GNU General Public License or GPL and permissive licenses such as the BSD licenses and the MIT License...

 and the Mozilla Public License
Mozilla Public License
The Mozilla Public License is a free and open source software license. Version 1.0 was developed by Mitchell Baker when she worked as a lawyer at Netscape Communications Corporation and version 1.1 at the Mozilla Foundation...

. Examples of non-copyleft free software licenses include the X11 license
MIT License
The MIT License is a free software license originating at the Massachusetts Institute of Technology . It is a permissive license, meaning that it permits reuse within proprietary software provided all copies of the licensed software include a copy of the MIT License terms...

, Apache license
Apache License
The Apache License is a copyfree free software license authored by the Apache Software Foundation . The Apache License requires preservation of the copyright notice and disclaimer....

 and the BSD licenses
BSD licenses
BSD licenses are a family of permissive free software licenses. The original license was used for the Berkeley Software Distribution , a Unix-like operating system after which it is named....

.

The Design Science License
Design Science License
Design Science License is a copyleft license for free content such as text, images, music and other content but not for documentation or source code. The DSL was written by Michael Stutz.-External links:*...

 is a strong copyleft license that can apply to any work that is not software or documentation, such as art, music, sports photography, and video. It is hosted on the Free Software Foundation website's license list, but it is not considered compatible with the GPL by the Free Software Foundation.

Full and partial copyleft


"Full" and "partial" copyleft relate to another issue: Full copyleft exists when all parts of a work (except the license itself) may only be modified and distributed under the terms of the work's copyleft license. Partial copyleft exempts some parts of the work from the copyleft provisions, thus permitting distribution of some modifications under terms other than the copyleft license, or in some other way does not impose all the principles of copylefting on the work. For example, the GPL linking exception made for some software packages (see below).

Share-alike


Share-alike
Share-alike
Share-Alike is a descriptive term used in the Creative Commons project for copyright licenses which include certain copyleft provisions. The Share-Alike license comes in two varieties, CC-BY-SA and CC-BY-NC-SA.-Share-alike license types:...

 imposes the requirement that any freedom that is granted regarding the original work must be granted on exactly the same or compatible terms in any derived work: this implies that any copyleft license is automatically a share-alike license, but not the other way around, as some share-alike licenses include further restrictions, for instance prohibiting commercial use. Some permutations of the Creative Commons licenses
Creative Commons licenses
Creative Commons licenses are several copyright licenses that allow the distribution of copyrighted works. The licenses differ by several combinations that condition the terms of distribution. They were initially released on December 16, 2002 by Creative Commons, a U.S...

 are examples of share-alike.

Viral licensing


Copyleft licenses are sometimes referred to as "viral license
Viral license
Viral license is a pejorative term used to describe a copyright license that allows derivative works only when licensed identically to the original. Licenses of this form include several common open source licenses, such as the GNU General Public License and the Creative Commons ShareAlike licenses...

s" because any works derived from a copyleft work must themselves be copyleft when distributed (and thus they exhibit a viral phenomenon
Viral phenomenon
Viral phenomena are objects or patterns able to replicate themselves or convert other objects into copies of themselves when these objects are exposed to them....

). The term 'General Public Virus', or 'GNU Public Virus' (GPV), has a long history on the Internet, dating back to shortly after the GPL was first conceived. Many BSD License
BSD licenses
BSD licenses are a family of permissive free software licenses. The original license was used for the Berkeley Software Distribution , a Unix-like operating system after which it is named....

 advocates used the term derisively in regards to the GPL's tendency to absorb BSD licensed code without allowing the original BSD work to benefit from it, while at the same time promoting itself as "freer" than other licenses. Microsoft
Microsoft
Microsoft Corporation is an American public multinational corporation headquartered in Redmond, Washington, USA that develops, manufactures, licenses, and supports a wide range of products and services predominantly related to computing through its various product divisions...

 vice-president Craig Mundie
Craig Mundie
Craig James Mundie is chief research and strategy officer at Microsoft. He started in its consumer platforms division in 1992, managing the production of Windows CE for hand-held and automotive systems and early console games. In 1997, Mundie oversaw the acquisition of WebTV Networks...

 remarked "This viral aspect of the GPL poses a threat to the intellectual property of any organization making use of it." In another context, Steve Ballmer
Steve Ballmer
Steven Anthony "Steve" Ballmer is an American business magnate. He is the chief executive officer of Microsoft, having held that post since January 2000. , his personal wealth is estimated at US$13.9 billion, ranking number 19 on the Forbes 400.-Early life:Ballmer was born in Detroit, Michigan to...

 declared that code released under GPL is useless to the commercial sector (since it can only be used if the resulting surrounding code becomes GPL), describing it thus as "a cancer that attaches itself in an intellectual property sense to everything it touches". The term 'viral' may be read as an analogy of computer virus
Computer virus
A computer virus is a computer program that can replicate itself and spread from one computer to another. The term "virus" is also commonly but erroneously used to refer to other types of malware, including but not limited to adware and spyware programs that do not have the reproductive ability...

es. According to FSF
Free Software Foundation
The Free Software Foundation is a non-profit corporation founded by Richard Stallman on 4 October 1985 to support the free software movement, a copyleft-based movement which aims to promote the universal freedom to create, distribute and modify computer software...

 compliance engineer David Turner, it creates a misunderstanding and a fear of using copylefted free software. David McGowan has written that there is no reason to believe the GPL could force proprietary software to become free software, but could "try to enjoin the firm from distributing commercially a program that combined with the GPL’d code to form a derivative work, and to recover damages for infringement." If the firm "actually copied code from a GPL’d program, such a suit would be a perfectly ordinary assertion of copyright, which most private firms would defend if the shoe were on the other foot."

Popular copyleft licenses, such as the GPL, have a clause allowing components to interact with non-copyleft components as long as the communication is abstract, such as executing a command-line tool with a set of switches or interacting with a Web server. As a consequence, even if one module of an otherwise non-copyleft product is placed under the GPL, it may still be legal for other components to communicate with it normally. This allowed communication may or may not include reusing libraries or routines via dynamic linking — some commentators say it does, the FSF asserts it does not and explicitly adds an exception allowing it
GPL linking exception
A GPL linking exception modifies the GNU General Public License to create a new, modified license. Such modified licenses enable software projects which provide library code, to be "linked to" the programs that use them, without applying the full terms of the GPL to the using program...

 in the license for the GNU Classpath
GNU Classpath
GNU Classpath is a project aiming to create a free software implementation of the standard class library for the Java programming language. Despite the massive size of the library to be created, the majority of the task is already done, including Swing, CORBA, and other major parts. The Classpath...

 re-implementation of the Java library.

One should also note that on one hand this 'viral' effect is a normal property of any conventional license on derived works of non-copyleft free material, and on the other hand it is the intended effect when using BSD-licensed works as part of proprietary software
Proprietary software
Proprietary software is computer software licensed under exclusive legal right of the copyright holder. The licensee is given the right to use the software under certain conditions, while restricted from other uses, such as modification, further distribution, or reverse engineering.Complementary...

. The GNU project using BSD code is in this respect no different from Microsoft or Apple using BSD code, despite claims by proponents of the GPL that it is a freer license than the BSD License.

Symbol


The copyleft symbol is a backwards C in a circle (copyright symbol
Copyright symbol
The copyright symbol, or copyright sign, designated by © , is the symbol used in copyright notices for works other than sound recordings . The use of the symbol is described in United States copyright law, and, internationally, by the Universal Copyright Convention...

 © mirrored). Because it is unavailable on Unicode
Unicode
Unicode is a computing industry standard for the consistent encoding, representation and handling of text expressed in most of the world's writing systems...

, it can be approximated with character or the more widely available character between parenthesis (ɔ) or, if supported by the application, by combining it with the character ↄ⃝. It has no legal meaning.

The Copyleft symbol can be included in HTML using following code (it only works for browsers that (fully or experimentally) support CSS3's "transform" property, or Microsoft's "BasicImage" filter):


©


See also



  • All rights reversed
  • Anti-copyright
    Anti-copyright
    Anti-copyright refers to the complete or partial opposition to prevalent copyright laws. Copyright is known as the owner's right for copies to be only made by the owner or with his/her authorization in form of a license....

  • Commercial use of copyleft works
    Commercial use of copyleft works
    Commercial advantage of copyleft works differs from traditional commercial advantage of Intellectual Property Rights . The economic focus tends to be on monetizing other scarcities, complimentary goods rather than the free content itself. One way to make money with copylefted works is to sell...

  • Copyright
    Copyright
    Copyright is a legal concept, enacted by most governments, giving the creator of an original work exclusive rights to it, usually for a limited time...

  • Creative Commons licenses
    Creative Commons licenses
    Creative Commons licenses are several copyright licenses that allow the distribution of copyrighted works. The licenses differ by several combinations that condition the terms of distribution. They were initially released on December 16, 2002 by Creative Commons, a U.S...

  • CrimethInc. N©! license
    CrimethInc. N©! license
    The CrimethInc. N©! license is a copyleft anarchist license with restrictions of use based on anti-statism and anti-corporatism. The terms of use were written by the CrimethInc. Ex-Workers Collective and included in some of their works such as the group's "cookbook", Recipes for Disaster. The...

     - a license that restricts use by governments and corporations
  • Free Art license
    Free Art license
    The Free Art License is a copyleft license that grants the right to freely copy, distribute, and transform creative works without needing the author's explicit permission.The Free Art License recognizes and protects these rights...

  • Free content
    Free content
    Free content, or free information, is any kind of functional work, artwork, or other creative content that meets the definition of a free cultural work...

  • Free Culture movement
    Free Culture movement
    The free culture movement is a social movement that promotes the freedom to distribute and modify creative works in the form of free content by using the Internet and other forms of media....

  • Free Software Foundation
    Free Software Foundation
    The Free Software Foundation is a non-profit corporation founded by Richard Stallman on 4 October 1985 to support the free software movement, a copyleft-based movement which aims to promote the universal freedom to create, distribute and modify computer software...

  • GNU General Public License
    GNU General Public License
    The GNU General Public License is the most widely used free software license, originally written by Richard Stallman for the GNU Project....

  • HESSLA
    Hacktivismo Enhanced-Source Software License Agreement
    The Hacktivismo Enhanced-Source Software License Agreement is a software license proposed by Hacktivismo that attempts to put ethical restrictions on use and modification of software released under it....

     - a copyleft license which prohibits uses that violate human rights or add spyware
  • Open content
    Open content
    Open content or OpenContent is a neologism coined by David Wiley in 1998 which describes a creative work that others can copy or modify. The term evokes open source, which is a related concept in software....

  • Open society
    Open society
    The open society is a concept originally developed by philosopher Henri Bergson and then by Austrian and British philosopher Karl Popper. In open societies, government is purported to be responsive and tolerant, and political mechanisms are said to be transparent and flexible...

  • Patentleft
    Patentleft
    Patentleft is the practice of licensing patents for royalty-free use, on the condition that adopters license related improvements they develop under the same terms...

  • Permissive free software license
  • Public domain
    Public domain
    Works are in the public domain if the intellectual property rights have expired, if the intellectual property rights are forfeited, or if they are not covered by intellectual property rights at all...

  • Share-alike
    Share-alike
    Share-Alike is a descriptive term used in the Creative Commons project for copyright licenses which include certain copyleft provisions. The Share-Alike license comes in two varieties, CC-BY-SA and CC-BY-NC-SA.-Share-alike license types:...

  • WTFPL
    WTFPL
    The WTFPL is an infrequently used, extremely permissive free software license. The original Version 1.0 license, released March 2000, was written by Banlu Kemiyatorn who used it for Window Maker artwork. Samuel “Sam” Hocevar, a French programmer who was the Debian project leader from 17 April...

     - a "free" license that is absolutely non-copyleft


External links