ITEMS IN A SERIES: The comma rule for separating three or more related items in a series would a no-brainer, were it not for one complication: the so-called serial comma.

INTRODUCTORY PHRASES AND CLAUSES: The comma rule for a group of related words that introduces the main idea of a sentence hinges on two factors: (1) the structure of the series; and (2) the number of words in the series.

INTERRUPTORS: A comma should both precede and follow any word, phrase, or clause that interrupts the flow of a sentence.

COMMA USE IN MULTI-CLAUSE SENTENCES:. The key to avoiding comma errors in sentences that include more than one clause is to recognize the difference between restrictive clauses and nonrestrictive clauses.