Okay, my age is showing. Please don’t laugh at me. The point is, commas can do a chameleon job on a sentence. For example:
The driver who won the race was awarded a trophy.
The driver, who won the race, was awarded a trophy.
In the first sentence, with no commas, any driver who won got a trophy. In grammatical terms, “who won the race” is restrictive. It restricts who gets the trophy.
In the second sentence, the driver, who just happened to win the race, got a trophy. In grammatical terms, “who won the race” is nonrestrictive. The driver got a trophy, and oh, by the way, also won the race.
Maybe it will make more sense in context:
1. The driver who won the race was awarded a trophy. The driver who came in second got a bottle of champagne. The other drivers received a dashboard plaque.
2. The driver, who won the race, was awarded a trophy. The pit crew was treated to a party. The team owner received a large check.
In example 1, “who won the race” specifies which driver. Only the winner gets a trophy. The paragraph concerns the rewards given to the participants in the race. In example 2, which concerns the rewards given to the members of a racing team, “who won the race” is just a bit of extra information and could be eliminated without changing the meaning.
Remember the example from last time? “I’d like you to meet my brother Dave.” The same rule applies. What this sentence means is, “I’d like to introduce you to my brother named Dave. My brother named Tom isn’t here.” Dave and Tom are restrictive because they specify which brother. (Not that either of my brothers has ever been able to restrict me. Hah.)
Got a headache yet? Don’t get too hung about the definition of restrictive vs. nonrestrictive. Just remember this--if the information in the phrase or clause could be deleted without changing the meaning of the sentence, you need to put commas around it. If the information specifies a particular person or thing, do not use commas.
Incidentally, another way to determine whether or not to use commas is to try putting dashes or parentheses around the words. If you could do so without changing the meaning, use commas. Be aware, however, that dashes are very emphatic, and parentheses are generally frowned on in fiction.
Cranky Old Grammar Lady, aka Nikki Andrews, is an editor at Champagne Books and a writer of mysteries and scifi. Visit her blog here for more grammar fun.