A
cross-reference is an instance within a
documentThe term document has multiple meanings in ordinary language and in scholarship. WordNet 3.1. lists four meanings :* document, written document, papers...
which refers to related or synonymous information elsewhere, usually within the same work.
To cross-reference or
to cross-refer (verb) is to make such connections. The term "cross-reference" is often abbreviated as
x-ref,
xref, or, in computer science,
XR. Cross-referencing is usually employed to either verify claims made by an author or to link to another piece of work that is of related interest. See, for example, the
LinuxLinux is a Unix-like computer operating system assembled under the model of free and open source software development and distribution. The defining component of any Linux system is the Linux kernel, an operating system kernel first released October 5, 1991 by Linus Torvalds...
cross-reference tool,
LXR for init/main.c.
In an
indexAn index is a list of words or phrases and associated pointers to where useful material relating to that heading can be found in a document...
, a cross reference is often denoted by
See Also. For example, under the term
Albert Einstein in the index of a book about Nobel Laureates, there may be the cross-reference
See Also: Einstein, Albert.
Hypertext
Cross-referencing in
hypertextHypertext is text displayed on a computer or other electronic device with references to other text that the reader can immediately access, usually by a mouse click or keypress sequence. Apart from running text, hypertext may contain tables, images and other presentational devices. Hypertext is the...
(
XR) is maintained internally or externally to a document with either in-context (XRIC) or out-of-context (
XROC) cross-referencing. These are analogous to KWIC and KWOC, which were very early computer applications inherited from the centuries-old idea of
concordanceConcordance can mean:* Concordance , a list of words used in a body of work, with their immediate contexts* Concordance , the presence of the same trait in both members of a pair of twins...
.
Traditionally, reference numbers and footnote marks are examples of in-context cross-referencing, whereas the
indexAn index is a list of words or phrases and associated pointers to where useful material relating to that heading can be found in a document...
and the reference list at the end of texts are examples of out-of-context cross-referencing. Out-of-context cross-referencing relies on the traditional, manually-produced indexes using subject or citation. This remained the mainstream text retrieval system until the advent of
CD-ROMA CD-ROM is a pre-pressed compact disc that contains data accessible to, but not writable by, a computer for data storage and music playback. The 1985 “Yellow Book” standard developed by Sony and Philips adapted the format to hold any form of binary data....
in 1985, since which the digital text, the
hypertextHypertext is text displayed on a computer or other electronic device with references to other text that the reader can immediately access, usually by a mouse click or keypress sequence. Apart from running text, hypertext may contain tables, images and other presentational devices. Hypertext is the...
, and eventually the
World Wide WebThe World Wide Web is a system of interlinked hypertext documents accessed via the Internet...
and search engines, provided systems for XRIC.
Soon after the advent of the Web, there was a rumor that XRIC features of the Web were better than the Gopher's XROC system.
Charles GoldfarbCharles F. Goldfarb is known as the father of SGML and is a co-inventor of the concept of markup languages. In 1969 Charles Goldfarb, leading a small team at IBM, developed the first markup language, called Generalized Markup Language, or GML. In an , Dr...
, one of the founding pioneers in SGML, satirically compared the antagonism between XROC and XRIC paradigms to a religious war, which would be moderately called the
cross-reference war. While the Web surpassed Gopher, in that XRIC is better than XROC in hypertext, both are as complementary as the two sides of the coin. Unfortunately, however, the
schismA schism , from Greek σχίσμα, skhísma , is a division between people, usually belonging to an organization or movement religious denomination. The word is most frequently applied to a break of communion between two sections of Christianity that were previously a single body, or to a division within...
between both text retrieval paradigms appears reflected on ACM/SIGIR and ACM/SIGWEB much overlapping each other.
The narrow or common sense of
hypertextHypertext is text displayed on a computer or other electronic device with references to other text that the reader can immediately access, usually by a mouse click or keypress sequence. Apart from running text, hypertext may contain tables, images and other presentational devices. Hypertext is the...
implies XRIC, while the wide or true sense includes XROC as well. From the text retrieval point of view,
hypertext as a new retrieval
paradigmThe word paradigm has been used in science to describe distinct concepts. It comes from Greek "παράδειγμα" , "pattern, example, sample" from the verb "παραδείκνυμι" , "exhibit, represent, expose" and that from "παρά" , "beside, beyond" + "δείκνυμι" , "to show, to point out".The original Greek...
, objecting to XROC or subjecting itself mainly to XRIC, sounds like a self-defeating misnomer, because
text retrieval and
cross-reference well comprise both XROC and XRIC in themselves. Ironically,
hypertext was coined by
Ted NelsonTheodor Holm Nelson is an American sociologist, philosopher, and pioneer of information technology. He coined the terms "hypertext" and "hypermedia" in 1963 and published it in 1965...
who used to object to the wide spectrum of text retrieval or
cross-reference and subject it mainly to the narrow idea of
transclusionIn computer science, transclusion is the inclusion of a document or part of a document into another document by reference.For example, an article about a country might include a chart or a paragraph describing that country's agricultural exports from a different article about agriculture...
, or simply
quotationA quotation or quote is the repetition of one expression as part of another one, particularly when the quoted expression is well-known or explicitly attributed by citation to its original source, and it is indicated by quotation marks.A quotation can also refer to the repeated use of units of any...
, aiming for text patchwork rather than retrieval.
RDBMS
A table can have an xref as prefix to indicate it is a cross-reference table that joins two tables together via primary key.
Lexicography
In printed and online dictionaries cross-references are important for several reasons. According to Nielsen (1999) they form a network structure of relations existing between different parts of data, dictionary-internal as well as dictionary external. The abstract mediostructure consists of all the possible sets of cross-referential relations. The actual realisation of these referential networks may be function-related, i.e. support a dictionary function such as translation. A distinction can be made between use-related and function-related cross-references. It is also possible to show hierarchical relationships (genus/species relation) between terms as well as sequential relations by using cross-references. The important point is that compilers of dictionaries need to take a broad approach to cross-references in dictionaries as they are directly linked to other structures in dictionaries.