That vs. which

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That
/ðət/ (Conj.)

Which
/wɪʧ/ (conj.)

That is a conjunction introducing a restrictive subordinate clause:

“I fed the cat that sat on the mat.” [implied: ... and I let the other one, which sat in the armchair, go hungry].

Which, by contrast, is a conjunction introducing a non-restrictive subordinate clause:

“The cat, which sat on the mat, had halitosis.” [Its halitosis was not contingent on its location mat-wise; indeed there could be an entirely distinct minty-breathed moggy sitting on the same mat, for all we know.]

Lesson: Get a dog.

See also

• Conjunction
• And/or