The Editor’s Manual
Free learning resource on English grammar, punctuation, usage, and style.
Use just one period, not two, after an abbreviation like “etc.” even when it appears in quotes at the end of a sentence. Don’t add another period after the closing quotation mark.
When an abbreviation like “etc.” appears in parentheses, place another period after the closing parenthesis to end the larger sentence. But if the entire sentence is enclosed in parentheses, use just one period, not two.
An acronym is an abbreviation pronounced as a single word. It is treated differently from other abbreviations (such as initialisms) in edited text.
Abbreviations are acceptable in formal writing. Explain an abbreviation at first use. Avoid overuse. Avoid using them in titles and headings or to start a sentence.
To form the plural of a contracted social or professional title, add “s” and move the period to the end (“Drs. Keyland and Murphy”). The plural of “Ms.” is “Mss.” or “Mses.,” and that of “Mr.” is “Messrs.”
Abbreviated SI and other metric units do not have a separate plural form: don’t add “s” to form the plural of an abbreviated unit of measurement (10 km, not 10 kms).
The abbreviations “UK” and “UN” are generally written without periods except in certain styles. The abbreviation “EU” is always written without internal periods.
The abbreviation for United States may be written with or without internal periods: U.S. or US. Both styles are acceptable.
When an abbreviation ending in a period is followed by a punctuation mark like a comma, colon, semicolon, or dash, use both the period and the punctuation mark. But use only a single period after an abbreviation at the end of a sentence.
If an abbreviation ending in a period also ends a sentence, use just one period, not two. With parentheses, place a period after the closing parenthesis to end the larger sentence.