Q. My question relates to the proper use of the comma when using a subordinate clause introduced by the pronoun “which” or “who.” My impression is that a comma to set off the clause is proper sometimes but not always. For example, if I say, “I have a car which has four doors,” a comma introducing the clause is not appropriate. However, if I say, “My car, which has four doors, is blue,” the comma is proper. I am not quite sure how to articulate the rule for when the comma is proper and when not. Can you help me?

A. Your instincts are right about the comma for nonrestrictive clauses—that is, clauses that are not necessary to the meaning of the sentence but are parenthetical. A good test is to put parentheses around the clause in question and see if any meaning is lost: if there is any change in the gist of the sentence, then you should not use the commas. When the clause is restrictive (that is, restricting or modifying the meaning of the subject, rather than simply adding to it), it is properly followed not by “which” but by “that”: I have a car that has four doors. Although this rule has now been relaxed almost out of existence, if you can get used to using “which” and “that” correctly, then you can follow the somewhat oversimplified guideline that “which” takes a comma.