Commas are grammar’s sheepdogs: they keep the various elements of a sentence from running off on their own and causing confusion and panic.
Commas on the Side
Commas are used to set apart non-essential pieces of your sentences. Words such as however and therefore need to be contained by a pair of commas to show that you’re interrupting your sentence with a bit of side business. Like an intermission, your aside may be interesting, but you don’t need it to understand the play.
- The Countess, however, refused to be seen in a Volkswagen.
- The chewing gum, therefore, was stuck to the ceiling.
- Many hobbies can be fulfilling, for example, stamp collecting, dental-floss macramé, and sugar-cube carving.
- The giant Madagascar hissing cockroach, I understand, makes a lovely companion.
- Kansas, she says, is the name of the star.
- The double-action shotgun, not diplomacy, was her forte.
- Quality food, including poutine, is best enjoyed after a night of heavy drinking.
This rule also applies when you’re addressing someone directly:
- Fernando, I don’t think you understand what this eggplant means to me.
- Grab a flamingo, everyone, and start playing!
- Dammit, Jim, I’m a doctor, not a go-go dancer!
Sometimes it’s not clear whether the phrase in question is side business or not. (This is when grammarians start throwing around the terms restrictive clause and nonrestrictive clause.) Take, for example, this sentence:
- My vampire, Duane, lives in the basement rec room.
This is fine if you only have one vampire; Duane can be removed from the sentence without changing its meaning. But what if you have a harem of vampires stashed in your house? You need the word Duane so your reader won’t think you’re talking about your other vampires, Joe, Hannah, and Billy Bob. When the phrase is needed to understand your sentence (that is, when it’s a restrictive clause), don’t use commas.
- My vampire Duane lives in the basement rec room. (I have multiple vampires.)
- My romance novel, Scottish Lords with Bulging Calf Muscles, is a real bodice-ripper. (This is my only romance novel. Take out the title and the sentence says the same thing.)
- My romance novel Scottish Lords with Bulging Calf Muscles is a real bodice-ripper. (I have written other romance novels, some of which feature other kinds of lords with different muscles. Remove the title and you won’t know which bodice-ripper I’m talking about.)
- She carried the shotgun not so much to shoot people as to scare them witless.
- If you insist on wandering in the woods during the full moon and are therefore bitten by a werewolf, don’t expect sympathy from me.
Sometimes grammarians talk about the that/which rule. This rule decrees that only necessary (restrictive) clauses use the word that, and only side business (nonrestrictive) clauses use the word which.
- The aliens that invaded my bathtub have used all the soap. (Not to be confused with the aliens in my pantry.)
- The alien craft, which landed on Tuesday, makes a striking lawn ornament. (Only one alien craft has ever landed in these parts.)
Of course, the British blithely ignore the that/which rule and use which whenever they damn well please. You can too, as long as you’re consistent.
Next week: commas and phrases and clauses, oh my!