The Elements of Style, Fourth Edition

Join two independent clauses with a colon if the second interprets or amplifies the first.

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.

Use a dash only when a more common mark of punctuation seems inadequate.

The colon also has certain functions of form: to follow the salutation of a formal letter, to separate hour from minute in a notation of time, and to separate the title of a work from its subtitle or a Bible chapter from a verse.

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

A colon may introduce a quotation that supports or contributes to the preceding clause.

It usually follows an independent clause and should not separate a verb from its complement or a preposition from its object.

A dash is a mark of separation stronger than a comma, less formal than a colon, and more relaxed than parentheses.

The number of the subject determines the number of the verb.