You already know that a comma is used to set off a parenthetical elaboration, as in
“Smoke on the Water,” a song by Deep Purple, has one of the most recognizable guitar riffs in the history of rock and roll.
In this case, “a song by Deep Purple” does not add to the meaning of the sentence; rather, it gives more information about its subject, which is the song. I almost never see these commas omitted in people’s writing, which is a good thing! Our English teachers would be proud of us all.
However, there are times in similar sentences when those commas are used but incorrect. Take this sentence:
Honolulu mayor, Mufi Hanneman, is a graduate of Harvard University.
People often put commas around “Mufi Hanneman” because they elaborate (or give more information about) the “Honolulu mayor” part of the sentence. However (and skip these next two sentences if grammatical mumbo-jumbo confuses you), in this case, the subject of the sentence is Mufi Hanneman and “Honolulu mayor” modifies THAT. In the first sentence, “a song by Deep Purple” modifies (and comes after) “Smoke on the Water.”
Using commas to offset Mufi Hanneman in the example above is kind of like using commas in this sentence:
A beautiful red, pickup truck, is one of my fondest memories of college.
So how can you tell when you need those commas and when you don’t? One quick trick is to REMOVE the words between the commas and see if the sentence still makes sense! Check this out:
“Smoke on the Water” has one of the most recognizable guitar riffs in the history of rock and roll.
Honolulu mayor is a graduate of Harvard University.
Because we took the stuff between commas out of the first sentence and it still makes sense, those words really were a parenthetical phrase and therefore should be set between commas. Because the second sentence makes NO sense without the words between commas, we know that those words were not a parenthetical phrase. Put the words back, but leave the commas out!
Occasionally, you’ll see a sentence like this:
My sister Catalina is difficult to get along with.
Does Catalina need to be between commas or not? Without Catalina, the sentence still makes sense: My sister is difficult to get along with. According to the advice I just gave you, then, you’d need to put commas around Catalina, right?
Wrong. This is one of those weird things about commas. Sometimes, the choice is yours. Because Catalina is a small word that doesn’t mess up the flow of the sentence when we don’t use the commas, you can actually skip it. It’s not wrong to use the commas, but if a sentence can be just as good without the commas, you are better off not using them. Most of us use commas too often anyway. Now, if your sister’s name were Catalina Magdalena Hobblesteiner Wobbleiner Hogan Logan Bogan, as in that great campfire song, you would definitely want to use the commas!