"That" introduces essential information; "which" introduces additional, non-essential information (set off by commas).
Introduces non-restrictive clauses (additional information removable without changing the core meaning). Set off by commas.
My car, which I bought last year, needs new tires already.
The Eiffel Tower, which was built in 1889, attracts millions of tourists every year.
The report, which took three weeks to complete, was well received by the board.
Introduces restrictive clauses (essential information needed to identify what is being discussed). No commas.
The car that you recommended turned out to be perfect for my needs.
I need a phone that has a long battery life.
The movie that won the Oscar last year is now available on streaming.
Common mistake
The rule in formal American English is clear: essential clause (no commas) = "that"; additional clause (with commas) = "which". In British English and informal speech the distinction is much more relaxed and both are used interchangeably.
Trick to remember it
The comma test: if you can put commas around the clause and the main sentence still makes complete sense — which. If removing the clause changes which thing you are talking about — that. Commas = which. No commas, identifying a specific one = that.
Quick rule
That = essential clause, no commas (defines which one we mean). Which = additional clause, with commas (extra information that can be removed).
Did you know?
This distinction was primarily codified by Fowler in his influential "A Dictionary of Modern English Usage" (1926). Before that, Shakespeare and other great writers used them interchangeably. Modern British English still frequently swaps them.
Exercise 1 · Pick the right one
I need to find a hotel _____ has a pool and is close to the city center.
Exercise 2 · Pick the right one
The Amazon River, _____ flows through Brazil, is the largest river in the world by discharge.
Exercise 3 · Form the word
That = essential clause, no commas (defines which one we mean). Which = additional clause, with commas (extra information that can be removed).
The rule in formal American English is clear: essential clause (no commas) = "that"; additional clause (with commas) = "which". In British English and informal speech the distinction is much more relaxed and both are used interchangeably.
The comma test: if you can put commas around the clause and the main sentence still makes complete sense — which. If removing the clause changes which thing you are talking about — that. Commas = which. No commas, identifying a specific one = that.
Bryan López
Founder of SpeakUP Academy