This document defines and compares defining and non-defining relative clauses. Defining relative clauses provide essential information about a noun, while non-defining clauses provide extra, non-essential information between commas. Defining clauses do not use commas and allow the word "that", while non-defining clauses require commas and do not allow "that". Common relative pronouns include who, which, whose, when, and where, with who referring to people and which referring to things. Examples are provided to illustrate the differences.

1. Defining relative clauses versus Non-defining relative clauses