parenthetical elements: when to use commas

A parenthetical element adds a comment, an aside, or a transition that stands apart from the main statement of the sentence. A phrase is not parenthetical merely because the sentence remains complete without the phrase. In Ana left after dinner, the phrase after dinner states when Ana left, so no comma precedes after dinner. Commas set off a parenthetical element: Our flight, as it turned out, left on time. A parenthetical element in the middle of a sentence takes a comma on each side. A parenthetical element at the beginning or the end of a sentence takes a single comma: In fact, the store closed early. The store closed early, in fact.

Function, not form, decides whether a phrase is parenthetical. In The plan, in my view, needs more work, the phrase in my view comments on the plan, so commas set the phrase off. In The keys are in my car, the phrase in my car states where the keys are, so no comma precedes the phrase.

Expressions such as however, of course, for example, and in fact are parenthetical when they comment on a sentence or link a sentence to the preceding sentence. When one of these expressions modifies a word in the sentence instead of commenting on the sentence, no commas set the expression off: However you travel, book early. In that sentence, however modifies travel and means “in whatever way.”

When however joins two independent clauses, a semicolon precedes however, and a comma follows it: The road was closed; however, the detour added only ten minutes.

See also appositives: comma use; comma with nonrestrictive clauses; parentheses vs. dashes vs. commas; conjunctive adverbs and semicolons; comma.