Punctuation Rules

Parallel Punctuation

Rule 6: All punctuation must be parallel.

This rule means that if you set off a free modifier that interupts a main clause with a comma at the beginning, you need to put one at the end of the free modifier as well. If you use a dash in one of those positions, you need to use it in the other too.

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

It also means that if you use a semicolon to set off one item in a list with a single sentence, then you must set off the other items in the list with semicolons as well.

parallel: We sent copies of the memo to Walla Walla, Washington; Traverse City, Michigan; and New Brunswick, New York.
broken parallel: We sent copies of the memo to Walla Walla, Washington, Traverse City, Michigan; and New Brunswick, New York.