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Lewis & Clark Style Guide

that, which

These words are not interchangeable in American English.

Which is used before a “nonessential” clause: The books, which are rare, are stored in a special room. (All of the books in question are stored in a special room. If you were to remove the words which are rare, the meaning of the sentence would not change.) A nonessential clause must be set off with commas.

That is used to introduce an “essential” clause: The books that are rare are stored in a special room. (Only the rare books are stored in a special room. Some of the books in question are not rare and are stored elsewhere. If you were to remove the words that are rare, the meaning of the sentence would change.) An essential clause must not be set off from the rest of the sentence by commas.

Hint: When in doubt, try the sentence both ways. If that fits comfortably, use it.

Lewis & Clark Style Guide

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