a9.1. A restrictive clause is one that is essential to the meaning of the sentence. It cannot be eliminated without changing what the sentence says. The man who set the fire got away: The clause who set the fire is essential, in that it specifies who the man was and why he would get away. Restrictive clauses are not set off with commas.
a9.2. A non-restrictive clause is one that is not essential to the basic meaning of the sentence. Its elimination does not change the author's intended message. Waldo, who was adorable, met a rather violent end for a mynah bird: The clause who was adorable simply tells us a bit more about Waldo; what the sentence is telling us, however, is about the violent end. Non-restrictive clauses are set off with commas.
a9.3. The relative pronoun that should be used only with restrictive clauses and should not be preceded by a comma: The sawmill that burned down last night belonged to the Martells. (Clause is restrictive because it specifies the sawmill being discussed.)
a9.4. The relative pronoun which should be used only with non-restrictive clauses and should always be preceded by a comma: The sawmill, which was more than 100 years old, burned down last night. (Clause is non-restrictive because it merely adds information to main thought of sentence.)
a9.5. Note importance of proper use of commas and pronouns to meaning of the sentence:
The sawmill that was more than 100 years old burned down last night. This sentence tells readers that there are several sawmills, out of which one burned down last night.
The sawmill, which was more than 100 years old, burned down last night. This sentence tells readers that the one sawmill they know about burned down last night. The fact that the sawmill is more than 100 years old is not essential to the main point of the sentence.
a9.6. Exception: Which may be used as a relative pronoun
introducing an essential clause in a sentence in which that is used
as a conjunction to begin another clause: Truman said that
the sawmill which burned down last night was more than 100 years
old. (This should be used only sparingly.)