The Editor's Manual
Grammar, usage, punctuation, and style resource for editors, writers, and learners of the English language.
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).
Both a colon and an em dash introduce new information that explains or builds upon something that precedes it. The colon is quieter; the dash is more emphatic and dramatic.
Use a colon only after a grammatically complete sentence to introduce a list. Don’t use a colon between a verb and its object. Don’t use a colon after a title, heading, or caption for a list.
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.
If an abbreviation ending in a period also ends a sentence, don’t add another period. Use a period after a closing parenthesis to end a sentence, but if the sentence with the abbreviation is entirely enclosed in parentheses, don’t add another period.
Use a colon to introduce a list or a quotation, or to explain and amplify a statement. It directs the reader’s attention to the information that follows. It can also serve as a sign or separator.
Use an apostrophe with possessives (New Year’s Day, Mother’s Day, Presidents’ Day) but not with descriptive terms (Veterans Day, Human Rights Day).
“It’s” (with the apostrophe) is a contraction of “it is” or “it has.” “Its” (without the apostrophe) is a possessive, used to show that one thing is related to another.
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.