prepositional phrases: when to use commas
When a prepositional phrase takes a comma.
A comma follows a long introductory prepositional phrase: After three weeks of talks with the landlord, the tenants signed the lease. The comma following a short introductory prepositional phrase is optional; keep the choice consistent within a piece: In June the pool reopens. In June, the pool reopens.
The comma is required, however, after a short introductory prepositional phrase when the noun that follows could be read as part of the phrase: Before leaving, the guests thanked the host. A reader of Before leaving the guests thanked the host could interpret the guests as the object of leaving until the verb thanked shows the error.
An inverted sentence takes no comma after its opening prepositional phrase: Under the bridge lived a family of otters.
A restrictive prepositional phrase that follows a noun takes no commas: The house on the corner sold in a week. A parenthetical prepositional phrase takes commas: The mayor, in a rare reversal, withdrew the proposal.
See also comma with introductory elements; parenthetical elements: when to use commas; participial phrases: comma use; commas between subjects, verbs, objects, and complements; comma.
Examples
UseDuring the first week of the trial, the jury heard from twelve witnesses. (A comma follows a long introductory prepositional phrase.)
UseThe keys on the counter belong to Marco. (No commas set off a restrictive prepositional phrase.)
UseThe museum, to our surprise, stayed open late. (Commas set off a parenthetical prepositional phrase.)
AvoidThe members of the board, approved the merger. (A comma wrongly follows the restrictive prepositional phrase of the board; a restrictive prepositional phrase takes no commas. Write The members of the board approved the merger.)
AvoidAcross the street, stood an old church. (A comma wrongly follows the opening prepositional phrase of an inverted sentence. Write Across the street stood an old church.)