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Q. Once and for all: to abbreviate “postscript” at the end of correspondence, is it best to write PS or P.S.? The glossary in CMOS advises no periods, but several examples in the Q&A use them, like this one. Help!
A. Chicago style for the abbreviation of “postscript” can be deduced from our recommendations for using periods with abbreviations (CMOS 10.4), which can be summarized as follows: (a) use periods for an abbreviation that consists of lowercase letters or that ends in a lowercase letter, such as p. or pp., a.m. or p.m., and Dr.; (b) use a period for an initial standing in for a name, as in E. B. White; but (c) omit periods from abbreviations that include two or more capital letters, such as US, PhD, and CEO.
So we would advise writing “PS,” no periods. (The lowercase alternative, “p.s.,” doesn’t seem to be supported by tradition. And the glossary entry you refer to is for the abbreviation of PostScript, the specialized programming language from Adobe—so that doesn’t count.)
As for our own use of “P.S.”—with periods—guilty as charged. As the Q&A has developed over the last twenty-plus years, Chicago style has evolved. Until 2003, we would have advised periods, but then we dropped them—first from academic degrees and most other abbreviations with capital letters (in the 15th ed.), and then also from “U.S.” (in the 16th ed.).
Another consideration: Postscripts are a little old-fashioned (you can usually go back and edit the body of your letter or email or whatever), and so are periods.
But that’s all in the past. Thanks to your query and others like it, we hereby announce “PS”—no periods—as our preference for “postscript.”
PS: You can follow “PS” with either a colon or a period. With our updated preference, the colon is best, but if you prefer “P.S.” you can leave it out.