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That and Which

“That” is restrictive; it is used to draw the reader’s attention to a specific aspect of the information you are giving. Because the information is essential, the clause is not set off with a comma. “Which” is non-restrictive, it is used to add information. This information is not essential to the meaning of the sentence, and therefore set off with a comma. For example: I will wear the dress that is in the laundry. I have more than one dress, but I want to wear that one.