Have you ever felt like Oscar Wilde? If so, this article can help.
"I have spent most of the day putting in a comma and the rest of the day taking it out."
Despite the difficulties many writers encounter, and the myriad rules describing their usage, commas have only two basic functions: they either separate or set off. Separating requires only one comma; setting off requires two.
A relative clause that is restrictive, that is, essential to the meaning of the sentence, should not be set off by commas. A nonrestrictive phrase or nonessential element is one that offers extra information but can be omitted without affecting meaning, and should be enclosed in commas, or if at the end of a sentence, preceded by a comma.
- The woman wearing a red coat is my sister.
- The report that the committee submitted was well documented.
- The book I have just finished is due back tomorrow; the others can wait.
- That is the woman who mistook my coat for hers.
- My sister, wearing a red coat, set off for the city.
- The report, which was well documented, was submitted to the committee.
- This book, which I finished last night, is due back tomorrow.
- The woman, who was extremely embarrassed, returned my coat.
Sources: Gregg Reference Manual and Chicago Manual of Style