Many people are confused about the exact usage of the relative pronouns that and which in North American English. Which is correct and in what context?
That is always used in a restrictive sense; that is, it defines or narrows a category or identifies an item in a group. A clause using the word that is necessary to the sentence or restricts the meaning. For example:
The picture that has the gilt frame is up for auction. In this example, we’re told specifically that it is only the picture with the gilt frame up for auction and presumably no others. The clause “that has the gilt frame” defines which picture is up for auction.
Which is nonrestrictive; that is, its clause is not necessary to or does not restrict the meaning of the sentence, but merely provides additional information about the subject. For example:
My aunt’s picture, which has a gilt frame, is up for auction. In this case, the clause “which has a gilt frame” is incidental to the main sentence; it’s just extra information and the sentence is not harmed if it’s removed.
If all this is too complicated to remember, there’s a simple rule of thumb that works in most cases. Which is almost always preceded by a comma, a dash, or a parenthesis, and that is not. You would not write, for example: My aunt’s picture, that has a gilt frame, is up for auction. The exception to this rule—isn’t there always one?—occurs when which is used restrictively and preceded by a preposition. E.g.: The picture in which many people showed interest was up for auction.
Note that these distinctions are sometimes not made in British English. It’s also interesting to note that you can get away with substituting which for that without too many grammarians writhing in discomfort, but in North American English, you shouldn’t substitute that for which. The above example, My aunt’s picture, that has a gilt frame, is up for auction, is clearly wrong.
And one final point. In a sentence like that which will not kill you will make you stronger, where that and which are side by side, it’s perfectly okay to use which as the relative pronoun. It sounds much better than that that will not kill you.
