An adjective clause is a dependent clause which takes the place of an adjective in another clause or phrase. Like an adjective, an adjective clause modifies a noun or pronoun, answering questions like "which?" or "what kind of?" Consider the following examples:

Adjective

the red coat

Adjective clause

the coat which I bought yesterday

Like the word "red" in the first example, the dependent clause "which I bought yesterday" in the second example modifies the noun "coat." Note that an adjective clause usually comes after what it modifies, while an adjective usually comes before.

In formal writing, an adjective clause begins with the relative pronouns "who(m)," "that," or "which." In informal writing or speech, you may leave out the relative pronoun when it is not the subject of the adjective clause, but you should usually include the relative pronoun in formal, academic writing:

informal

The books people read were mainly religious.

formal

The books that people read were mainly religious.

informal

Some firefighters never meet the people they save.

formal

Some firefighters never meet the people whom they save.

Here are some more examples of adjective clauses:

the meat which they ate was tainted

This clause modifies the noun "meat" and answers the question "which meat?".

about the movie which made him cry

This clause modifies the noun "movie" and answers the question "which movie?".

they are searching for the one who borrowed the book

The clause modifies the pronoun "one" and answers the question "which one?".

Did I tell you about the author whom I met?

The clause modifies the noun "author" and answers the question "which author?".