contrasting phrases: comma use
The commas that set off a phrase that states a contrast, such as one beginning with not.
A comma precedes a contrasting phrase introduced by not: The flight leaves from Gate 12, not Gate 21. A contrasting phrase in the middle of a sentence takes a comma on each side: The interview is on Tuesday, not Wednesday, at ten o’clock.
A contrast built on not . . . but takes no comma, because the two parts form a single paired construction: The delay was caused not by the weather but by a broken signal.
See also comma; correlative conjunctions and parallelism; not only . . . but also; comma before a coordinating conjunction.
Examples
Use Linda ordered the salmon, not the steak. (A comma precedes a contrasting phrase introduced by not.)
Use The concert starts at eight, not seven, on Saturday. (A contrasting phrase in the middle of a sentence takes a comma on each side.)
Use The problem is not the price but the delivery time. (A contrast built on not . . . but takes no comma.)
Avoid The tickets cost forty dollars not thirty. (A comma must follow dollars, because a comma precedes a contrasting phrase introduced by not. Write The tickets cost forty dollars, not thirty.)
Avoid The game is on Friday, not Thursday at the high school. (A comma must follow Thursday, because a contrasting phrase in the middle of a sentence takes a comma on each side. Write The game is on Friday, not Thursday, at the high school.)