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.