Commas set off non-restrictive clauses
Many writers feel doubt about the need for commas around relative clauses.The way to overcome this doubt is to understand the difference between restrictive and non-restrictive clauses. A restrictive (or defining) clause is essential to identify the noun it modifies.
For instance, in the sentence "The boy who stole the bicycle was caught and punished," the restrictive clause (shown in boldface type) is necessary to the sense of the subject, and the sentence. In other words, the boy who stole the bicycle is not just any boy.
On the other hand, if we look at the slightly altered sentence "Tommy, who stole the bicycle, was caught and punished," we can see at once that the name Tommy identifies the specific boy. In this sentence, the phrase "who stole the bicycle" is not necessary to define or identify who is the subject of the sentence.
Commas represent pauses in speech, and when we speak this sentence aloud, we pause before and after this non-restrictive modifier, and when we write the sentence down, we mark these pauses with commas. The pauses, and the commas that represent them, clarify the meaning of the sentence.
The non-restrictive adjective clause is not the only type of modifier that requires commas before and after. Adverb clauses and other modifiers, sometimes called sentence interrupters, may also preceded and followed by commas, as the following sentences show.
The sky, they say, is the limit.
The moon, we were told, is made of green cheese.
The sentence, we noticed, contained an improperly punctuated restrictive clause.
The theatre, unfortunately, was not the only building that had to close due to the fire.
These interrupter commas travel in pairs, one before and one after the modifier, unless, of course, the restrictive clause reaches the end of the sentence. In that case, it is preceded by a comma and followed by a period. Here is an example:
I finally got a chance to see the old movie, which I had never seen before.
A word to the wise: To master comma usage, the writer must understand sentence structure thoroughly, including the relationship between the comma on paper and the pause in speech.
Ready for a quiz? Try one by Jane Straus here. Need more comma rules? Check where to use commas here and where NOT to use commas here.