Use a comma to separate items in a series of three or more words, phrases, or clauses.

I bumped into professors, horizontal bars, agricultural students, and swinging iron rings.

—JAMES THURBER, “University Days”

He has plundered our seas, ravaged our coasts, burnt our towns, and destroyed the lives of our people.

—THOMAS JEFFERSON, Declaration of Independence

You may see a series with no comma after the next-to-last item, particularly in newspaper writing. Occasionally, however, omitting the comma can cause confusion.

Without the comma after peas, you wouldn’t know if the cafeteria offered three vegetables (the third being a mixture of peas and carrots) or four.

When the items in a series contain commas of their own or other punctuation, separate them with semicolons rather than commas (55b).

Coordinate adjectives, those that relate equally to the noun they modify, should be separated by commas.

In a sentence like The cracked bathroom mirror reflected his face, however, cracked and bathroom are not coordinate because bathroom mirror is the equivalent of a single word, which is modified by cracked. Hence, they are not separated by commas.

You can usually determine whether adjectives are coordinate by inserting the word and between them. If the sentence still makes sense with the and, the adjectives are coordinate and should be separated by commas.

They are sincere and talented and inquisitive researchers.

The sentence makes sense with the ands, so the adjectives should be separated by commas: They are sincere, talented, inquisitive researchers.

The sentence does not make sense with the ands, so the adjectives should not be separated by commas: Byron carried an elegant gold pocket watch.