The Editor’s Manual
Free learning resource on English grammar, punctuation, usage, and style.
Commas always go inside quotation marks in American style. In British style, commas go inside or outside depending on whether they punctuate the text within quotes or the surrounding sentence.
You may either omit or include the comma after greetings like “Hi” and “Hello” in informal email salutations, depending on preference.
Use either a comma or a colon after a greeting. The colon is preferred in business or other formal communication in U.S. style.
Use commas to integrate closely related information into the flow of the sentence. Use parentheses to set off supplementary information or an afterthought from surrounding text. Use dashes to be emphatic or dramatic and make additional information or an aside stand out.
Don’t use a comma before “who” when it presents information necessary to meaning (a restrictive clause). Do use a comma when “who” introduces an optional description (a nonrestrictive clause).
Don’t use a comma with “that,” either as a relative pronoun or a conjunction. “Which” usually introduces an optional description, which you should enclose in commas. No commas are used if “which” introduces essential information.
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.
Use commas before and after the year in the American date format (May 1, 2021) but not in British (1 May 2021). No comma is needed when only the month and year are specified (May 2021).
The serial comma is used before the conjunction (“and,” “or”) that marks the final item in a series. Using it is optional, but it can sometimes affect the meaning of a sentence.
Use commas to make lists, set off phrases, separate clauses, and indicate that a detail is nonessential in a sentence.