What is a relative clause ?

A relative clause is a clause that modifies a noun.  A relative clause is often called an adjective clause because it functions as an adjective in a sentence.

A relative clause is introduced by a relative pronoun, a relative adverb, or a zero relative.

Relative pronouns are which, that, who, whom, whose.

e.g.  She is the person to whom I have sent the letter.

Relative adverbs are where, when, why.

e.g.  I will always remember the day when I met her.

A zero relative is occuring when we omit the relative pronoun.

e.g.  The man you hired  is not very responsible. (the relative pronoun is left out because it is the object of the relative clause)

c. a group of tribes that formed a confederacy