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When I was in school, strictly every abbreviation had a period after each (capitalized) letter, as in U.S.A. These days, it seems ok just to capitalise, as in USA.

Is this a new rule? It does look jarring to have those periods everywhere, and it's annoying to type.

Related: Omitting periods after title abbreviations (Mr, Mrs, Dr).

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I'm inclined to believe it's related to the length of the abbreviation. (For example, I've seen "U.S." and "USA" occur separately in the same article.) – Ben Blank Feb 17 '11 at 0:55
up vote 8 down vote accepted

It depends on the type of abbreviations. USA is an initialism, and as such, does not require periods. The style of acronyms and initialisms no longer requiring periods is now becoming more prevalent.

Abbreviations, generally, still require periods (e.g. Prof. — Oh, and e.g.) Although there are exceptions, especially in British english usage, as noted in your linked related question.

Which abbreviations take periods and which do not is variable depending on the style guide followed.

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I agree that abbreviations still require abbreviations. And periods, too. ;o) – deceze Feb 17 '11 at 2:34

Lowercase initialisms like e.g., i.e., n.a., and q.v. generally retain their periods to help distinguish them from words (or, in the case of n.a. and relatives, inexplicably take a slash in the middle instead).

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