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.

2. When one or both of the clauses are short or closely related in meaning, the comma is often omitted.

   They said good-bye and everyone hugged.

If commas set off another phrase that modifies the whole sentence, the comma between main clauses is often omitted.

   Six thousand years ago, the top of the volcano blew off in a series of powerful eruptions and the sides collapsed into the middle.

3. Commas are sometimes used to separate short and obviously parallel main clauses that are not joined by conjunctions.

   One day you're a successful corporate lawyer, the next day you're out of work.

Use of a comma to join clauses that are neither short nor obviously parallel, called comma fault or comma splice, is avoided. Clauses not joined by conjunctions are normally separated by semicolons.

4. If a sentence is composed of three or more clauses that are short and free of commas, the clauses are occasionally all separated by commas even if the last two are not joined by a conjunction. If the clauses are long or punctuated, they are separated with semicolons; the last two clauses are sometimes separated by a comma if they are joined by a conjunction.

   Small fish fed among the marsh weed, ducks paddled along the surface, an occasional muskrat ate greens along the bank.

   The kids were tired and whiny; Napoleon, usually so calm, was edgy; Tabitha seemed to be going into heat, and even the guinea pigs were agitated.