Nonrestrictive phrases or clauses are not essential to the basic meaning of the sentence. They provide additional information; readers don’t require the information in order to understand what the writer is trying to say. We enclose that information with commas.
- Joanne, who often plays golf, is going to the opera.
Restrictive phrases and clauses are those which are necessary in the sentence because they restrict, or limit, the meaning of the sentence. Restrictive phrases and clauses are not enclosed with commas. Look at the following example:
- People who love metal will want to go to the Pantera concert.
Read the sentence as though “who love metal” is a nonrestrictive (non-essential) clause. If we enclose it with commas, we’re saying that you can remove those words without changing the meaning of the sentence. If we remove the words, the sentence reads:
- People will want to go to the Pantera concert.
Does removing the words change the meaning? Yes; it implies that all people will want to go. “Who love metal” is definitely a restrictive phrase. It limits the people who will want to go to the Pantera concert to those people who love metal.
Look at these examples:
- People who drive with their headlights off at night must be crazy.
- George, who often drives with his headlights off at night, must be crazy.
In the first sentence, “who drive with their headlights off at night” is restrictive; it’s necessary for the basic meaning of the sentence. Without that restrictive clause, we’d be saying
- People (meaning all people) must be crazy.
In the second sentence, we’re saying that George is crazy. “Who often drives with his headlights off at night” is just one example – some extra information – to tell us why George must be crazy. If you remove the clause from the sentence. George is still crazy.