AI Tutor
Imagine a biology teacher announces: "The lab samples that were contaminated have been destroyed." Now imagine they add two commas: "The lab samples, which were contaminated, have been destroyed."
In the first sentence, only a specific batch was ruined; in the second, every single sample in the lab was contaminated and thrown away.
How can a pair of punctuation marks completely flip the factual scope of your sentence? It all comes down to the mechanics of relative clauses.

The Anatomy of a Relative Clause

A relative clause is a dependent clause that modifies a noun or pronoun, functioning just like an adjective. It begins with a relative pronoun (who, whom, whose, which, that) or a relative adverb (where, when, why).
When the relative pronoun serves as the object of a preposition in formal prose, we often place the preposition directly in front of it—creating constructions like in which, to whom, or by whose authority.
Now, how do you decide whether a relative clause needs commas or not?

Restrictive Clauses: The Essential Filter

A restrictive clause delivers information essential to identifying which specific noun you mean. If you remove a restrictive clause, the core meaning of the main clause changes or becomes confusingly vague.
Because this information is structurally vital, we never separate a restrictive clause with commas. In standard American English, we use that for things and who or that for people.