The Editor's Manual
Free learning resource on English grammar, punctuation, usage, and style.
Commas and periods always go inside quotes in American style. In British style, they go inside only if they apply to the quoted text. In all styles, question marks and exclamation points go inside only if they apply to the quoted text.
Periods always go inside quotation marks in American usage. In British usage, a period (or full stop) goes inside only if the quotation is a complete sentence.
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.
The abbreviation for United States may be written with or without internal periods: U.S. or US. Both styles are acceptable: AP and APA style is to use periods (U.S.); Chicago, MLA, and CSE style is to omit them (US).
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.
Periods are used when the abbreviations for time are lowercased (a.m., p.m.) but omitted when they are capitalized (AM, PM).
An ellipsis is a series of three dots. In a quotation, an ellipsis signifies omitted words. In a dialogue or narrative, the ellipsis shows faltering speech or a pause.
A period indicates a longer pause than a comma or a semicolon. Use a period to end a sentence and to mark the end of a thought or fragment. Also use periods in abbreviations and to separate initials before a surname.
Periods are not used in abbreviations with more than one capital letter, but contractions and other abbreviations with lowercase letters may contain a period (rtc., p.m., Dr.). No period is used after an abbreviated SI unit (kg, m).
Accepted guidelines: Provide the full form of an abbreviation at first use. Avoid using abbreviations at start of sentence. Use “a” or “an” depending on how the abbreviation is pronounced. And more.