A three-letter acronym, three-letter abbreviation, or TLA is an abbreviation, specifically an acronym, alphabetism, or initialism, consisting of three letters. These are usually the initial letters of the words of the phrase abbreviated, and are written in capital letters (upper case); three-letter abbreviations such as etc. and Mrs. are not three-letter acronyms.
Most three-letter abbreviations are initialisms: all the letters are pronounced as the names of letters, as in APA/ˌeɪpiːˈeɪ/AY-pee-AY. Some are acronyms pronounced as a word; computed axial tomography, CAT, is almost always pronounced as the animal's name in "CAT scan". When TLA stands for three-letter abbreviation, then TLA has the self-referential feature that TLA is its own TLA (and is thus autological). When TLA stands for three-letter acronym, this feature does not apply (as it is an abbreviation but not an acronym).
The exact phrase three-letter acronym appeared in the literature in 1975. Three-letter acronyms were used as mnemonics in biological sciences, and their practical advantage was promoted by Weber in 1982. They are used in many other fields, but the term TLA is particularly associated with computing. The specific generation of three-letter acronyms in computing was mentioned in a JPL report of 1982.
In 1980, the manual for the Sinclair ZX81 home computer used and explained TLA. In 1988, in a paper titled "On the cruelty of really teaching computer science", eminent computer scientist Edsger W. Dijkstra wrote "Because no endeavour is respectable these days without a TLA ..." By 1992 it was in a Microsoft handbook.
Use of "TLA" spread through both industry and academia, and it has now become a generally understood initialism.
The number of possible three-letter abbreviations using the 26 letters of the alphabet from A to Z (AAA, AAB ... to ZZY, ZZZ) is 26 × 26 × 26 = 17,576. Another 26 × 26 × 10 = 6760 can be produced if the third element is allowed to be a digit 0-9, giving a total of 24,336.
In English, WWW is the longest possible TLA to pronounce, typically requiring nine syllables. Although in written English it is an abbreviation, in spoken English it ordinarily uses more syllables than the words it is abbreviating.
References in popular culture
As early as 1967, the musical Hair included the song "Initials", whose final verse consisted only of TLAs, viz: "LBJ IRT USA LSD. LSD LBJ FBI CIA. FBI CIA LSD LBJ."
In 1986, Will Shatter of the band Flipper formed a band named "Any Three Initials" (A3I), as a parody of the preponderance of hardcore punk bands with three-initial names.
In 1998, the British band Love and Rockets released their last album, Lift, featuring the song "R.I.P. 20 C." that, apart from the refrain, consists only of three-letter abbreviations. A contest was held rewarding the first person to correctly give the meanings of all 69 of them.
In 1999, German hip-hop group Die Fantastischen Vier (The Fantastic Four) released the song "MfG" ("Mit freundlichen Grüßen", German for "Best regards", literally "With friendly greetings"), also mainly consisting of TLAs.
In 1999, the author Douglas Adams remarked: "The World Wide Web is the only thing I know of whose shortened form takes three times longer to say than what it's short for."
In 2001, Portland Oregon songwriter Craig Carothers produced a song entitled "BFD" which includes many three-letter acronyms throughout the lyrics. It has been recorded by Carothers as well as Kathy Mattea, Berkley Hart and Don Henry.
According to the Jargon File, a journalist once asked hacker Paul Boutin what he thought the biggest problem in computing in the 1990s would be. Paul's straight-faced response was: "There are only 17,000 three-letter acronyms."
The Jargon File also mentions the abbreviation "ETLA" for "extended three letter acronym" to refer to four letter acronyms/abbreviations. Also, "extended three letter acronym" is sometimes abbreviated to "XTLA".
The ska band TLA  from Dunedin, New Zealand, uses "TLA" to mean "two-letter acronym".
^Seavey, S. R.; Raven, P. H. (1977). "Chromosomal Differentiation and the Sources of the South American Species of Epilobium (Onagraceae)". Journal of Biogeography4 (1): 57. JSTOR3038128. "All taxa indicated by three-letter acronyms with strains indicated by a fourth letter if necessary."
^Weber, W. A. (1982). "Mnemonic Three-Letter Acronyms for the Families of Vascular Plants: A Device for More Effective Herbarium Curation". Taxon31 (1): 74–88. JSTOR1220592.
^Nilsen, K. D.; Nilsen, A. P. (1995). "Literary Metaphors and Other Linguistic Innovations in Computer Language". The English Journal84 (6): 65–71. JSTOR820897.
^TDA Progress Report R. Hull (1982) An Introduction to the new Productivity Information Management System page 176
^Steven VickersZX81 Basic Programming, Sinclair Research Limited, page 161 "As you can see, everything has a three letter abbreviation (TLA)."