Relative clauses give extra information about a noun. They begin with relative pronouns like who, whom, whose, which, that. Knowing the difference between defining and non-defining clauses is key to correct punctuation.

Who vs. Which vs. That

• Who / Whom: Used strictly for human beings. (The girl who won the prize...)
• Which: Used for animals, things, and non-defining descriptive clauses. (The car, which was red, broke down.)
• That: Used for essential defining clauses for both people and things. (The phone that I bought yesterday is lost.)

Punctuation: Commas or No Commas?

Defining clause: Essential to identify the noun. NO commas. (Students who work hard pass the exam.)

Non-defining clause: Extra bonus info. MUST be enclosed in commas. (My brother, who lives in London, is a doctor.)