16. Abbreviations and periods

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.