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The rules for the restrictive/nonrestrictive use of commas complement those for the use of which vs. that. In formal writing, use that to introduce a restrictive clause, or one that is essential to the meaning of the sentence. Use which to introduce a nonrestrictive clause, or one that does not add essential information. Use commas only with which clauses.
The suit that I bought for Joe's wedding is the only one that has ever fit me perfectly.
Now, I need some shoes that will go with my suit.
My old dress shoes, which have been gathering dust in the closet, are the wrong style and color.
I need to buy the shoes by Friday, which is the day before the wedding.
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