Defining and Non-Defining Relative Clauses
A relative clause gives us information about the noun it modifies. A defining relative clause gives essential information about the noun it modifies. The sentence would not make sense if the clause were removed. Defining relative clauses often come right after the nouns that they modify, without a comma.
People who eat healthy foods live longer.
What’s the name of the hotel that you stayed at?
A non-defining relative clause gives us additional but non-essential information about the noun it modifies. The sentence would still make sense if the non-defining clause were removed. In writing, non-defining relative clauses are usually separated from the rest of the sentence by commas before and after the clause.
Isabel Allende, who wrote Daughter of Fortune, lives in the San Francisco Bay Area.
Watership Down, which is my favorite novel, is a story about rabbits.