A comma is sometimes used to set off contrasting expressions within a sentence.
- This project will take six months, not six weeks.
When two or more contrasting modifiers or prepositions, one of which is introduced by a conjunction or adverb, apply to a noun that follows immediately, the second is set off by two commas or a single comma, or not set off at all.
A solid, if overly wordy, assessment
- or a solid, if overly wordy assessment
- or a solid if overly wordy assessment
This street takes you away from, not toward, the capitol.
- or This street takes you away from, not toward the capitol.
grounds for a civil, and maybe a criminal, case
- or grounds for a civil, and maybe a criminal case
- or grounds for a civil and maybe a criminal case
Dashes or parentheses are often used instead of commas in such sentences.
- grounds for a civil (and maybe a criminal) case
A comma does not usually separate elements that are contrasted through the use of a pair of correlative conjunctions such as either . . . or, neither . . . nor, and not only . . . but also.
- Neither my brother nor I noticed the error.
- He was given the post not only because of his diplomatic connections but also because of his great tact and charm.
When correlative conjunctions join main clauses, a comma usually separates the clauses unless they are short.
- Not only did she have to see three salesmen and a visiting reporter, but she also had to prepare for next day's meeting.
- Either you do it my way or we don't do it at all.
Long parallel contrasting and comparing clauses are separated by commas; short parallel phrases are not.
- The more that comes to light about him, the less savory he seems.
- The less said the better.