The comma is the most frequently used punctuation mark in English and the one that provides the most difficulties to writers. Its most common uses are to separate items in a series and to set off or distinguish grammatical elements within sentences.

Between Main Clauses

1. A comma separates main clauses joined by a coordinating conjunction, such as and, but, or, nor, or so.

   She knew very little about the new system, and he volunteered nothing.

   The trial lasted for nine months, but the jury took only four hours to reach its verdict.

   We will not respond to any more questions on that topic this afternoon, nor will we respond to similar questions in the future.

   All the first-floor windows were barred, so he had clambered up onto the fire escape.