Punctuation of Adjective Clauses

General Guidelines:
1. DO NOT USE COMMAS IF the adjective clause is necessary to identify the noun it modifies.*
2. USE COMMAS IF the adjective clause simply gives additional information and is not necessary to identify the noun it modifies.**

Examples:

a) The professor who teaches Chemistry 101 is an excellent lecturer.
b) Professor Wilson, who teaches Chemistry 101, is an excellent instructor.

In a): No commas are used. The adjective clause is necessary to identify which professor is meant.
In b): Commas are used. The adjective clause is not necessary to identify who Professor Wilson is. We already know who he is: he has a name. The adjective clause simply gives additional information.

c) Hawaii, which consists of eight principal islands, is a favourite vacation spot.
d) Ms. Batte, who is a retired teacher, writes poetry and children’s stories.

Guideline: Use commas, as in b), c) and d), if an adjective clause modifies a proper noun. (A proper noun begins with a capital letter, not a lower case letter.)
Note: A comma reflects a pause in speech.

e) The woman who(m) I met teaches English.
The woman that I met teaches English.
The woman I met teaches English.

f) Ms. Batte, whom I met yesterday, teaches English.

In e): If no commas are used, any possible pronoun may be used in the adjective clause. Object pronouns may be omitted.
In f): When commas are necessary, the pronoun that may not be used (only who, whom, which, whose, where, and when may be used), and object pronouns cannot be omitted.

COMPARE THE MEANING
g) We took some children on a picnic. The children, who wanted to play soccer, ran to an open field as soon as we arrived at the park.
h) We took some children on a picnic. The children who wanted to play soccer ran to an open field as soon as we arrived at the park. The others played a different game.

In g): The use of commas means that all of the children wanted to play soccer and all of the children ran to the open field. The adjective clause is used only to give additional information about the children.
In h): The lack of commas means that only some of the children wanted to play soccer. The adjective clause is used to identify which children ran to the open field.







* Adjective clauses that do not require commas are called “essential” or “restrictive” or “identifying.”
** Adjective clauses that require commas are called “nonessential” or “nonrestrictive” or “nonidentifying.”
Note: Nonessential clauses are more common in writing than in speaking.