When abbreviating something, such as the United States, when did it become acceptable to abbreviate it to US instead of U.S.?
Abbreviations can be shortened versions of words (etc.) which we pronounce in full (et cetera); they can be acronyms, which are pronounceable (UNESCO, AIDS); or they can be initialisms, which require us to pronounce every letter. In common acronyms and initialisms, the periods tend to drop out: F.B.I. becomes FBI, R.C.M.P. becomes RCMP, and so on.
This is a matter of changing usage, and it's still perfectly OK (or O.K.) to keep the periods in. But in our impatient age, when every keystroke counts, many people prefer to drop the periods.
This is not universal, however. The Canadian Press Stylebook, used by many journalists here, recommends keeping periods in geographical abbreviations and initials of persons:
B.C., U.S., L.A.; J.R. Ewing, E.T.
But it also acknowledges that well-known public figures may become known by initials without periods: JFK, FDR, W.
The Stylebook also offers the following advice on punctuating abbreviations:
Write compound abbreviations without spaces:
Omit periods from currency abbreviations:
$500 US, $800 Cdn
Put periods in most lower and mixed abbreviations:
f.o.b., Jr., lb., Mrs., m.p.h., B.Comm.
But metric symbols are technically not abbreviations and they don't take periods except at the end of a sentence:
His weight is 80 kg.
Mixed abbreviations that begin and end with a capital letter do not take periods:
PhD, PoW, MiG, U of T
Single-letter abbreviations take a period:
127 E.(for East) Fifth Street
Letters themselves, however, don't take a period:
Brand X, the letter J