Dashes, like commas, semicolons, colons, ellipses, and parentheses, indicate added emphasis, an interruption, or an abrupt change of thought. Experienced writers know that these marks are not interchangeable. Note how dashes subtly change the tone of the following sentences:

You are the friend, the only friend, who offered to help me.
You are the friend—the only friend—who offered to help me.
I pay the bills; she has all the fun.
I pay the bills—she has all the fun.
I wish you would … oh, never mind.
I wish you would—oh, never mind.

Rule 1. Words and phrases between dashes are not generally part of the subject.

Example: Joe—and his trusty mutt—was always welcome.

Rule 2. Dashes replace otherwise mandatory punctuation, such as the commas after Iowa and 2013 in the following examples:

Without dash: The man from Ames, Iowa, arrived.
With dash: The man—he was from Ames, Iowa—arrived.
Without dash: The May 1, 2013, edition of the Ames Sentinel arrived in June.
With dash: The Ames Sentinel—dated May 1, 2013—arrived in June.

Rule 3. Some writers and publishers prefer spaces around dashes.

Example: Joe — and his trusty mutt — was always welcome.