The Editor’s Manual
Free learning resource on English grammar, punctuation, usage, and style.
Use parentheses (or round brackets) to set off supplementary information from surrounding text. Such information should not be essential to the grammar of the sentence. Periods go within parentheses if they belong to the parenthetical matter but outside otherwise.
Use “who” as a relative pronoun to link a description to the person it describes. “Who” is used not just for people but also animals with names. “Who” can replace “whom” in informal usage.
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.
Use “which” to introduce a description. As a relative pronoun, “which” connects a relative clause to the noun it describes. Differences exist between American and British usage.
Use “that” as a relative pronoun in restrictive or defining clauses, which present information essential to meaning. Don’t use a comma before “that.”
Use commas to make lists, set off phrases, separate clauses, and indicate that a detail is nonessential in a sentence.
Use a comma between the items in a list, to separate two independent clauses, and after a subordinate clause. Enclose nonessential phrases and clauses in commas.