The Elements of Style, Fourth Edition

Enclose parenthetic expressions between commas.

Use a dash to set off an abrupt break or interruption and to announce a long appositive or summary.

Use a colon after an independent clause to introduce a list of particulars, an appositive, an amplification, or an illustrative quotation.

Note that if the second clause is preceded by an adverb, such as accordingly, besides, then, therefore, or thus, and not by a conjunction, the semicolon is still required.

In a series of three or more terms with a single conjunction, use a comma after each term except the last.

A colon tells the reader that what follows is closely related to the preceding clause. The colon has more effect than the comma, less power to separate than the semicolon, and more formality than the dash.

No comma, however, should separate a noun from a restrictive term of identification.

1. Form the possessive singular of nouns by adding 's.

A comma is preferable when the clauses are very short and alike in form, or when the tone of the sentence is easy and conversational.