Acronym

====Showing the ellipsis of letters====
In English, abbreviations have previously been marked by a wide variety of [[English punctuation|punctuation]]. Obsolete forms include using an [[overbar]] or [[colon (punctuation)|colon]] to show the [[ellipsis]] of letters following the initial part. The [[forward slash]] is still common in many dialects for some fixed expressions{{mdash}}such as ''w/'' for "with" or ''A/C'' for "[[air conditioning]]"{{mdash}}while only infrequently being used to abbreviate new terms. The [[apostrophe]] is common for [[Contraction (grammar)#English|grammatical contractions]] (e.g. ''don't'', ''[[y'all]]'', and ''[[ain't]]'') and for contractions marking unusual pronunciations (e.g. ''a'ight'', ''cap'n'', and ''fo'c'sle'' for "all right", "captain", and "forecastle"). By the early twentieth century, it was standard to use a [[full stop|full stop/period/point]], especially in the cases of initialisms and acronyms. Previously, especially for [[Latin abbreviations]], this was done with a full space between every full word (e.g. {{lang|la|A. D.}}, {{lang|la|i. e.}}, and {{lang|la|e. g.}} for "[[Anno Domini]]", "[[id est]]", and "[[exempli gratia]]"). This even included punctuation after both [[Roman numerals|Roman]] and [[Arabic numerals]] to indicate their use in place of the full names of each number (e.g. ''LII.'' or ''52.'' in place of "fifty-two" and "1/4." or "1./4." to indicate "one-fourth"). Both conventions have fallen out of common use in all dialects of English, except in places where an Arabic [[decimal number|decimal]] includes a medial [[decimal point]].

Particularly in [[British English|British]] and [[Commonwealth English]], all such punctuation marking acronyms and other capitalized abbreviations is now uncommon and considered either unnecessary or incorrect. The presence of all-capital letters is now thought sufficient to indicate the nature of the ''[[United Kingdom|UK]]'', the ''[[European Union|EU]]'', and the ''[[United Nations|UN]]''. Forms such as ''the U.S.A.'' for "the [[United States of America]]" are now considered to indicate [[American English|American]] or [[North American English]]. Even within those dialects, such punctuation is becoming increasingly uncommon.<ref>[[Oxford English Dictionary]]: ''initialism''. "Globe & Mail (Toronto) 22 May 10/4 Americanization has also largely done away with periods in acronyms and initialisms."</ref>

=====Ellipsis-is-understood style=====
Some [[style guide]]s, such as that of the [[BBC]], no longer require punctuation to show [[ellipsis]]; some even proscribe it. [[Larry Trask]], American author of ''The [[Penguin Books|Penguin]] Guide to Punctuation'', states categorically that, in [[British English]], "this tiresome and unnecessary practice is now obsolete."<ref>{{cite web |url= http://www.informatics.susx.ac.uk/doc/punctuation/node28.html |title=Abbreviations |publisher=Informatics.Susx.ac.uk |access-date=September 16, 2010 |archive-url= https://web.archive.org/web/20070410113359/http://www.informatics.susx.ac.uk/doc/punctuation/node28.html |archive-date=April 10, 2007 }}</ref>

=====Pronunciation-dependent style and periods=====
Nevertheless, some influential [[style guide]]s, many of them [[American English|American]], still require periods in certain instances. For example, ''[[The New York Times Manual of Style and Usage]]'' recommends following each segment with a period when the letters are pronounced individually, as in "[[KGB|K.G.B.]]", but not when pronounced as a word, as in "[[NATO]]".<ref name = "NY Times">{{Cite news |url= https://www.nytimes.com/2004/02/07/opinion/secret-obsessions-at-the-top.html |access-date=October 10, 2015 |title=Secret Obsessions at the Top |first=Nicholas D. |last=Kristoff |work=[[The New York Times]] |date=February 7, 2004 |archive-url= https://web.archive.org/web/20150528024321/http://www.nytimes.com/2004/02/07/opinion/secret-obsessions-at-the-top.html |archive-date=May 28, 2015 |url-status=live}}</ref> The logic of this style is that the pronunciation is reflected graphically by the punctuation scheme.

=====Other conventions=====
When a multiple-letter abbreviation is formed from a single word, periods are in general not used, although they may be common in informal usage. "TV", for example, may stand for a ''single'' word ("television" or "transvestite", for instance), and is in general spelled without punctuation (except in the plural). Although "PS" stands for the single English word "[[postscript]]" or the Latin ''postscriptum'', it is often spelled with periods ("P.S.") as if parsed as Latin ''post scriptum'' instead.

The [[slash (punctuation)|slash]] ('/', or ''solidus'') is sometimes used to separate the letters in an acronym, as in "N/A" ("not applicable, not available") and "c/o" ("care of").

Inconveniently long words used frequently in related contexts can be represented according to their letter count as a [[numeronym]]. For example, "i18n" abbreviates "[[internationalization]]", a computer-science term for adapting software for worldwide use. The "18" represents the 18 letters that come between the first and the last in "internationalization". "Localization" can be abbreviated "l10n", "[[multilingualization]]" "m17n", and "[[accessibility]]" "a11y". In addition to the use of a specific number replacing that many letters, the more general "x" can be used to replace an unspecified number of letters. Examples include "Crxn" for "crystallization" and the series familiar to physicians for [[medical history|history]], [[medical diagnosis|diagnosis]], and [[therapy|treatment]] ("hx", "dx", "tx"). Terms relating to a command structure may also sometimes use this formatting, for example gold, silver, and bronze levels of command in UK policing being referred to as Gx, Sx, and Bx.