abbreviations: periods

In recent years there has been a trend toward the omission of periods in abbreviations. This is particularly true of scientific and technical writing, but the practice has been spreading in general writing as well. The guidelines below will help you to use periods correctly.

Do not use periods with

chemical symbols and mathematical abbreviations: H2O, NaCl, cos, log, tan;
SI symbols and units: cm, kg, L;
abbreviations for points of the compass: winds NNW
(Exception: In street addresses that are not on an envelope or package, use periods with compass point abbreviations: King St. E.);
the military rank abbreviations used in the Department of National Defence;
short forms of words: lab, flu, vet, stereo, typo;
abbreviations or acronyms consisting exclusively of upper-case letters or ending in an upper-case letter (except those for personal names, legal references and most place names): e.g. NAFTA, PhD, YMCA, UN, GST, MiG, CTV.

Use periods

with geographical abbreviations, e.g. B.C., P.E.I., but not for the two-character symbols recommended by Canada Post.

(Note: While periods are traditional with geographical abbreviations, there is a growing tendency to omit these periods; thus, abbreviations like B.C. or P.E.I. can also be correctly written without periods: BC, PEI.)

with most lower-case abbreviations, including a.m., p.m., e.g., i.e. (mph is one of the few exceptions).
at the end of abbreviations for single words: Mr., Jr., Ltd., misc., pp., Nos.
after each abbreviated word of a multiword term or phrase, where the abbreviation of each word consists of more than single initials, e.g. Rev. ed., Rt. Rev. (Note that a space is required after each element in the abbreviation.)
after initials in a person’s name:
- Thelonius S. Monk
- H. E. Hughes

(Note that a space is required between each period and the following initial or name.)

Note

If a sentence ends in an abbreviation taking a period, only one period is used.