We use non-defining relative clauses to provide extra information about someone or something. This is not necessary information, meaning that we don't need it to understand who or what is being referred to.
English
Rule 1: We always use a relative pronoun (who, which, whose, or whom) to introduce a non-defining relative clause.
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Rule 2: In contrast to defining relative clauses, we don’t use that to introduce a non-defining relative clause.
| Mary, that we met at the party, was the only nice person there. | INCORRECT |
| Mary, who we met at the party, was the only nice person there. | CORRECT |
Note: In writing, we use commas around non-defining relative clauses. In speaking, we often pause at the beginning and end of the clause.
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Quiz
1/4Choose the right relative pronoun to introduce a non-defining relative clause. Yesterday I met Lucy, ___ mother lives in Bristol.