Parenthetical Expressions

Parenthetical expressions are words that can be taken out of a sentence, leaving behind a complete sentence. It may not be the sentence you wanted, but it's still complete.

If you can put the words in parentheses, or remove them entirely, then you need two commas -- just like you'd need opening and closing parentheses.

Everyone hates Red Impulse.
Everyone , except Ken, hates Red Impulse.
Everyone , except Ken, who is his son, hates Red Impulse.
Everyone , except Ken, who is his son, hates, or even despises, Red Impulse, the infamous pilot .

Sometimes it's a matter of style.

George ran fast.
George ran, fast.

The first one is most common; it describes how George ran. In the second one, George is running; "fast" is an afterthought -- it draws attention to the speed.

If the expression is short, it doesn't always need the commas -- but use either both or none, not one.

It can also change the sense of a sentence.

My aunt Tracy sews.
My aunt, Tracy, sews.

The first one, you are calling her "Aunt Tracy". The second one, you are specifying which aunt -- Tracy.

Note the period after "Aunt Tracy". Americans would put it inside. Brits usually put it inside, but in this case, because the period is not part of the quotation, it goes outside. More on that under Quotation Marks.

That's it for Parenthetical Expressions. Strunk and White say you can (and maybe should) leave them off for short things, but use either both or none. Lynne Truss says using both is a sign of being too cowardly to choose which to leave off.

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