The comma, that curved little wisp of a punctuation mark, indicates a pause. It gives readers a chance to catch their breath as they navigate the ideas in a sentence. Commas also help shape sentences, defining which words go together.
Commas tend to travel in pairs, especially in sentences containing nonessential clauses and appositives, nouns (or noun phrases) sitting rights next to other nouns that they rename, identify or define. Note that the sentence I’ve just written contains an appositive: . . . appositives, nouns (or noun phrases) . . . (So does the lede sentence of this post.) Note, too, that the phrase is set off by a comma — just one in this case, since the appositive ends the sentence. If it were in the middle of a sentence, as in my lede, there would be two. (You can also start a sentence with an appositive as a way to vary your structures.)
Often, though, you’ll see an appositive without a comma. Correct or incorrect? That depends. The basic rule: if there’s only one of the noun described by the appositive, set it off with commas. If there’s more than one, no commas.
An example from student work this semester:
Yoko Ono’s son Sean Lennon
Does Yoko have more than one son? Not to my knowledge. So it should be:
Yoko Ono’s son, Sean Lennon, . . .
What about my mother Edna or my wife Anne? A person has only one mother, so it should be my mother, Edna, . . . Similarly, one can have only one wife at a time, so it’s my wife, Anne, — unless, of course, the speaker is a polygamist or has been married multiple times (in which case it would help the reader to identify some of the wives as ex-).
Suppose you were writing about my dog Spot. Comma or none? That depends on whether you have only one dog, whose name is Spot, or more, one of which happens to be named Spot. If Spot’s an only dog, it’s my dog, Spot, . . . If Spot is one of several, then it’s my dog Spot, as opposed to your dog Fido or your dog Fifi. (The writer Willie Morris got it wrong in the title of his book “My Dog Skip” and the subsequent movie.)
In appositives, commas work pretty much the way they do in relative clauses. You will recall from That? Which? WHAT?!?!? that a nonessential clause calls for which and is set off by commas. So if you have just one, my dog, Spot, is equivalent to my dog, whose name is Spot, . . . But my dog Spot means my dog whose name is Spot, not Fifi or Fido.
Parenthetical expressions are by nature nonessential and thus should be set off by commas:
The pork chop, however, was one of the best I have ever eaten.
Another good place to use commas is before the conjunction (and, but, or, etc.) in compound sentences, as in:
The sun is out today, and it’s very warm.
The comma isn’t absolutely necessary there, and some copy editors will take it out. But notice how it gives your reader a slight break between two connected ideas.
A comma before certain conjunctions changes their meaning. When a comma precedes for, as or since, the word means because. (Except for emphasis, you rarely need a comma before because, since because already means because. And notice how I just used since, preceded by a comma, to mean because.) So:
It’s very warm today, for the sun is out. (A rather old-fashioned use of for.)
It’s very warm today, since the sun is out. (But no comma if since refers to time: It’s been very warm today since the sun came out.)
It’s very warm today, as the sun is out. (Same here, if as means while: It’s turned very warm as the sun has come out.)
It’s very warm today because the sun is out. No comma.
Commas have meaning; they should never be sprinkled randomly over a piece of writing. Learn the rules. The AP Stylebook entry on commas is a good place to start.