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.Notice that a dash is two hyphens typed without spaces before, after, or between them.
They drove to Ottawa that Tuesday morning--and, when they arrived at the courthouse, they nervously sought a parking place.
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
4c. A dash can't set off an initial-position
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