When do adjective clauses require a comma to introduce them?

Question

Sometimes I see sentences in which a comma precedes the adjective clause, as in: The doctor, who treated my illness, has retired. Other times, I see it with no comma, as in: The doctor who treated my illness has retired.

Is there an easy rule to help me remember when to use a comma?

Answer 1

In the first sentence, there is only one doctor, and the reader is aware of the doctor. In the second sentence, there are multiple doctors, and you are specifying which doctor retired.

The answer depends on whether or not the adjective clause is an IDENTIFYING CLAUSE.

Here is another example:

My sister who lives in Chicago will visit.

In this sentence, I have more than one sister. The sister who will visit me is the one who lives in Chicago, not my other sisters who live in other cities. If there is NO comma before the adjective clause, it means the clause is identifying WHICH sister will visit.

My sister, who lives in Chicago, will visit.

In this sentence, I only have one sister. We know this because of the commas. The commas tell us that this is a NON-IDENTIFYING CLAUSE, which means the clause is not necessary to determine which sister will visit...it is extra information. "My sister" alone identifies who I am talking about; I must only have one sister.

The rule is: if the clause is necessary for the reader to identify which specific noun you are describing, then don't use commas. If the clause provides extra information and is not necessary to identify the specific noun, use the commas.