A FLAB is an abbreviation
An abbreviation is a shortened form of a word or phrase. Usually, but not always, it consists of a letter or group of letters taken from the word or phrase...

 using four letters. It is an abbreviation of Four Letter ABbreviation. It extends the self-referential humour of the TLA (Three Letter Acronym). It is sometimes referred to as an ETLA or XTLA (Extended Three Letter Acronym)


In a world where every possible Three Letter Acronym has been registered as a .com
The domain name com is a generic top-level domain in the Domain Name System of the Internet. Its name is derived from commercial, indicating its original intended purpose for domains registered by commercial organizations...

, and almost every pronounceable one has many usages, there is a lot more scope for a unique name and greater readability with four letters. Using only upper-case English letters, there are 264 = 456,976 possible four-letter abbreviations. If numbers, special characters, or case-sensitivity are allowed, many more FLABs are available.

FLABs are commonly used as codes within computer file format
File format
A file format is a particular way that information is encoded for storage in a computer file.Since a disk drive, or indeed any computer storage, can store only bits, the computer must have some way of converting information to 0s and 1s and vice-versa. There are different kinds of formats for...

s and computer protocols where a compromise needs to be found between human readability and a fixed format. Many file formats such as TIFF and PNG use four-letter codes to identify the type of chunks. The developers of the Apple Macintosh operating system used four-letter abbreviations as type code
Type code
A type code is the only mechanism used in pre-Mac OS X versions of the Macintosh operating system to denote a file's format, in a manner similar to file extensions in other operating systems. Codes are four-byte OSTypes...

s and creator code
Creator code
A creator code is a mechanism introduced in pre-Mac OS X versions of the Macintosh operating system to link a data file to the application program which created it, in a manner similar to file extensions in other operating systems. Codes are four-byte OSTypes. For example, the creator code of the...

s. The Simple Mail Transfer Protocol
Simple Mail Transfer Protocol
Simple Mail Transfer Protocol is an Internet standard for electronic mail transmission across Internet Protocol networks. SMTP was first defined by RFC 821 , and last updated by RFC 5321 which includes the extended SMTP additions, and is the protocol in widespread use today...

 (SMTP) introduced the idea of using four-letter abbreviations as codes for commands (e.g. HELO, QUIT).

In some of these contexts, the FLAB is called a FourCC
A FourCC is a sequence of four bytes used to uniquely identify data formats.The concept originated in the OSType scheme used in the Macintosh system software and was adopted for the Amiga/Electronic Arts Interchange File Format and derivatives...

 (an abbreviation of Four Character Code) or OSType
OSType is the name of a four-byte sequence commonly used as an identifier in Mac OS. While the bytes can have any value, they usually display figures characterized in software programs such as those utilized in ASCII or Mac OS Roman character sets.OSType values are used to identify file data...


See also

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