英语语法：120 Dashes, Parentheses, and Commas

By Mignon Fogarty

Today I'll explain the difference between dashes, commas, and parentheses 1.

Stever Robbins, the host of the Get-It-Done Guy podcast, has been writing a book, and his birthday is today. He told me that his birthday wish would be for me to do a show to help him understand the difference between dashes, commas, and parentheses because it keeps coming up in his writing. Well, Stever, it's an odd thing to want for your birthday, but here it is.

In general, you can think of parentheses, commas, and dashes as a continuum of marks. Parentheses are the quiet whisper of an aside, commas are the conversational 2 voice of a friend walking by your desk, and dashes are the yowl of a pirate dashing into a fray 3.

Parentheses

Let's start with those quiet parentheses. You use them to surround something that seems a bit out of place in the sentence--an aside, a clarification, or a commentary. Sometimes when you go back to edit your first draft, you'll find that you can rework your sentence to include the parenthetical statement or simply delete the things in parentheses, unless they're something like irreverent quips that are an intentional 4 part of your tone.

Here's an example of one way to use parentheses to add additional information:

The 30th anniversary of the eruption 5 of Mount St. Helens (May 18, 1980) brought back vivid memories of ash and darkness. 

The date (May 18, 1980) is in parentheses in that sentence. It's something you want to tell the reader, but it isn't a necessary part of the sentence. If you leave it out, the reader still gets the whole point you wanted to make about revived memories because of the anniversary.

The date isn't enough of a dramatic statement to merit dashes, and if you want to leave it in, another good reason to use parentheses is that the date already contains a comma between the day and the year, so to surround it with commas would make the sentence difficult to read. No excitement. Already has an internal comma. That leaves parentheses as the obvious choice.

Here's one that's a little different:

I'm heading out (movie night!), but I'll call you in the morning.

"Movie night" is more of an aside or comment than a clarification. "Movie night" is so far removed from the flow of the sentence that you wouldn't want to use commas around it. You could use dashes. It doesn't seem like enough of an interruption or a dramatic statement to me to merit dashes, but it's a judgment 6 call. You could write the sentence a different way, of course, "I'm heading out for movie night, but I'll call you  in the morning," but it doesn't have the same friendly, happy feel. Parentheses seem right here.

The examples I've given both have sentence fragments enclosed in parentheses, but you can also enclose whole clauses.

Dashes

At the other end of the spectrum 7, we have dashes. If you want to hang a spotlight 8 on your words, decorate them with dashes. You can use dashes the same way we just talked about using parentheses, to enclose fragments or whole sentences, but you'd better be sure your words are worthy 9 of dashes. Dashes interrupt your sentence in a way that parentheses or commas don't. Here's an example:

They fled through the woods, and then George--dear, sweet George the accountant--jumped out from behind a tree and stabbed them.

It's appropriate to interrupt that sentence with dashes to remind the reader that the attacker has unexpected qualities--that he's dear, sweet George the accountant.

But this is English, so there's an exception to the dashes-are-dramatic rule. You can use dashes in a mundane 10 sentence when the part you need to set off already has commas, like the date we enclosed in parentheses earlier. You could write

The 30th anniversary of the eruption of Mount St. Helens--May 18, 1980--brought back vivid memories of ash and darkness.

Just remember that when you use dashes instead of parentheses, you're highlighting the date instead of simply noting it or providing it as background information.

Another difference between parentheses and dashes is that you always have to use two parentheses, they always enclose something, but it's fine to use one dash alone to introduce an important or exciting statement, or a statement that already has commas in it. You could write

There was only one thing missing from the pirate ship--pirates.

That dash is appropriate because the announcement that the pirates are missing is probably important or dramatic. In a sentence like that, where something is defined or expanded, you're choosing between the dash and a colon 11. You could just as properly write

There was only one thing missing from the pirate ship: pirates.

That sentence just doesn't have the same wild feeling as the sentence with the dash. A colon is a more stoic 12, buttoned up punctuation 13 mark than a dash.

And as before, you can also use one dash to introduce a longer, pedestrian statement if the statement already contains commas.

Commas

So let's finish with commas. They're kind of dull, which means you should always consider using them because punctuation usually shouldn't be drawing attention to itself. There are probably a hundred different rules that govern how to use commas, so I'm going to limit this discussion to commas that could be used like parentheses or dashes.

Commas don't interrupt your sentence, so you use them when the words you're enclosing are a natural part of your sentence and not some comment from left field or flamboyant 14 statement. Commas are generally used for appositives, for example, which are defining or clarifying statements after nouns. Here's an example of an appositive set off with commas:

My youngest sister, Meghan, will be visiting soon.

"Meghan" just tells you who my youngest sister is. You could set her name off with dashes as we did in the earlier sentence about George the accountant, or with parentheses like we did with a date earlier, but there's no reason to in a sentence like this one.

Commas are also used to set off non-restrictive elements such as "which" clauses.

Diamonds, which are expensive, aren't something I buy very often.

Like a parenthetical, the "which" clause could be left out of the sentence without changing the meaning. I actually did a whole episode just about "which" versus 15 "that" and commas, so you can read if you'd like to learn more.

Summary

As I'm sure you've noticed by now, you could make a legitimate 16 argument for using at least two different punctuation marks in nearly every example sentence I've given you, but these general rules may be helpful:

Use parentheses when you want to enclose something that is incidental to the sentence, something that is background or almost unnecessary. 

Use dashes when you want to enclose or set off something that deserves a lot of attention, is meant to interrupt your sentence, or already has commas or parentheses in it. 

Use commas to enclose things that belong firmly in the flow of your sentence. 

I know it can be frustrating 17 that there aren't hard-and-fast rules about when to use commas, parentheses, or dashes, but learning to use your judgment is part of finding your voice and becoming a better writer. In this case, the rules are more like the pirates' code in Pirates of the Caribbean--they're more what you'd call guidelines than actual rules. I bet Jack 18 Sparrow never used a comma; he seems like a dash man to me.