The Elements of Style, Fourth Edition

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

Follow this rule whatever the final consonant.

Restrictive clauses, by contrast, are not parenthetic and are not set off by commas.

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

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

"Write in a way that comes naturally," "Revise and rewrite," "Do not explain too much," and the rest; above all, the cleansing, clarion "Be clear."

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

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

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.