Punctuation Rules

The Dash

A dash (--) is like a strong comma, but it cannot separate all the items in a list--only the last--and it cannot set off initial-position free modifiers.

4a. Since it is like a comma, it can do the things that a comma can do, such as setting off free modifiers from the main clause and joining two main clauses with the help of a coordinate conjunction. It is a MORE EMPHATIC piece of punctuation than a comma, and so it is used to produce emphasis.

The athletes--marching in as a group--smiled excitedly as they looked up at the stadium full of spectators.

They drove to Ottawa that Tuesday morning--and, when they arrived at the courthouse, they nervously sought a parking place.

Notice that a dash is two hyphens typed without spaces before, after, or between them.

4b. A dash can't separate all the items in a list; however, it can set off the last item in a list for emphasis.

On our farm we grow wheat, soybeans, alfalfa, corn--and bamboo!
4c. A dash can't set off an initial-position free modifier.
wrong: When we could stand it no longer--we called to find out when they were coming.
The only time you can do that is when the main clause following the initial-position free modifier begins with "these."
okay: Hot dogs, apple pie, and Mom--these are the traditional American symbols.
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