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which and that

Standard American English uses "which" for nonrestrictive or nondefining phrases and clauses and "that" for restrictive or defining phrases and clauses. The word "which" usually signals the approach of added, nonessential information. When a phrase or clause is not essential to the meaning of a sentence, use the relative pronoun "which" and enclose the phrase or clause in commas. See also nonrestrictive phrases and clauses and restrictive phrases and clauses.

This paper, which she has been working on for three weeks, discusses string theory.

When a phrase or clause is essential to the meaning of a sentence (that is, the sentence would not make much sense without it), use "that" and leave out the commas.

The paper that he completed recently will be presented in New York; the paper that he finished last summer will be presented in Philadelphia.