During a grammar lesson, an English teacher presented two sentences to contrast defining and non-defining relative clauses: (1) "The students who passed the exam celebrated together" and (2) "My professor, who has published several books, retired last year." She explained that in one sentence the relative clause provides essential information to identify the antecedent, while in the other it adds supplementary information about an already-identified noun — a distinction signaled by punctuation. The sentence containing a non-defining relative clause requiring commas is: