A clause is a group of words that contains a subject and a verb. Some clauses are dependent: they can't stand alone and need an independent clause, or sentence, to support them.

These dependent clauses can be used in three ways: as adjectives, as adverbs and as nouns. This article focuses on adjective clauses.

What is an adjective clause?

An adjective clause is a dependent clause that acts as an adjective. That is, it modifies the noun or pronoun that comes before it:

Dr. Bondar, who went into space in 1992, was our first female astronaut.
(clause modifies noun Dr. Bondar)

Anyone who is afraid of doing too much will end up doing too little.
(clause modifies pronoun anyone)

What words are signs of an adjective clause?

Most adjective clauses start with the relative pronouns which, who and that:

Which is used for animals or things:

The spaniel barked at the squirrel, which ran up a tree for safety.

The squirrel dropped a large acorn, which landed on the spaniel's head.

Who is used for people (and occasionally for animals seen as persons):

Everyone who went on the cruise had a great time.

My cat Lulu, who loves catnip, will enjoy this catnip mouse.

Who can change its form to whom (when it's an object) or whose (to show ownership):

Sarah Jacobs, whom we met at the art show, is a well-known sculptor.

The man whose bike was stolen needs a ride home.

Note that we can avoid the awkward phrase of which by applying whose to animals or things:

The ferret whose leash broke is running in the park.
(not the leash of which broke)

Luke towed away the car whose battery had died.
(not the battery of which had died)

That can apply to people, animals or things:

The mayor congratulated the firefighter that won the award.

The dog that ate my socks has indigestion.

We enjoy the carnival that comes to town every summer.

Note that we sometimes omit that when it is the object of a verb or preposition:

The girl Jason took to the party is a close friend of Kristen's.
(= that Jason took to the party; that is object of verb took)

I can't find the envelope I put the cheque in.
(= that I put the cheque in; that is object of preposition in)

Are there any other words that can begin an adjective clause?

Yes, the relative adverbs when, where and why can start an adjective clause:

Noon is the hour when the sun is overhead.
(clause modifies noun hour)

The shop where we bought the camera was near the bridge.
(clause modifies noun shop)

I don't know the reason why Michel was late.
(clause modifies noun reason)

When does an adjective clause need commas?

The use of commas depends on whether the clause is essential to the meaning of the sentence.

Essential clauses don't take commas:

The woman who hosted the party is my cousin.

Here, the clause who hosted the party is essential because it identifies the woman. Without that clause, we would not know which woman the writer meant. Because the clause is essential to the meaning of the sentence and cannot be omitted, we don't set it apart with commas.

Non-essential clauses take commas:

Alana Bishop, who hosted the party, is my cousin.

The sentence Alana Bishop is my cousin would be clear by itself. The clause who hosted the party is just extra information that may be interesting but is not essential. Since the clause is merely giving secondary information that could be omitted, we set it off with commas.

Note that with essential clauses referring to animals or things, we normally use that, although which is also acceptable:

The books that (or which) I borrowed from the library are overdue.

However, with non-essential clauses, we must use which for animals or things and who for people:

Andy's parrot, which is a cockatiel, is called Kiki.

Please send a schedule to Mr. Bashan, who has joined our aerobics class.