This is “Relative Pronouns and Clauses”, section 1.16 (from appendix 1) from the book Writers' Handbook (v. 1.0).
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An adjective clause gives information about a preceding noun in a sentence. Look at the following examples.
The car that Richie was driving was yellow.
Des Moines, where I live, is in Iowa.
Mr. Creeter, whose brother I know, is the new math teacher.
Like many other adjective clauses, these begin with a relative adjective (which, who, whom, whose, that) or a relative adverb (when or where). When you use a relative clause to describe a noun, make sure to begin it with one of the seven relative adjectives and adverbs listed in the previous sentence.