Essential and Nonessential Adjective Clauses

Adjective Clause-a clause that functions as an adjective in the sentence. It modifies a noun by telling us which one or what kind. Usually begin with a relative pronoun: who, whom, that, which, whose). Can begin with a relative adverb: why, when, where.
Examples: The boy who is talking is named Kevin. (modifies "boy)
The picture that is hanging on the wall is beautiful. (modifies "picture")
The reason why I will not go is that I am sick. (modifies "reason")
Some adjective clauses are essential to the meaning of the sentence:
Example: The picture that is hanging on the wall is beautiful. (you need to know which picture)
Some adjective clauses are nonessential-meaning they provide extra information.
Example: My teacher, who is a football fanatic, wears red to school a lot. (you do not have to know she is a football fanatic to understand the rest of the sentence)
Nonessential clauses should be set off with commas.