"Where" Versus "In Which"
There's a time and a place for everything.
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The other reason you might want to use “which” is to avoid using a preposition at the end of a sentence, which is considered informal (3). For example, the sentence “This is the desk in which the papers are stored” is probably more appropriate to say in a formal setting than its informal counterpart: “This is the desk that the papers are stored in.” Some sticklers might object to that informal sentence. However, if you are talking informally, it is fine to end a sentence with a preposition or to use “where” instead of “in which,” as here: “This is the desk where the papers are stored.”
When to Use “Where”
Think back to the puppy sentence we said at the beginning. You probably wouldn’t say “the pound at which” to a friend in normal conversation. You would most likely say, “The pound where I found Spot was on State Street.” You might also find yourself saying, “I found Spot at the State Street pound,” bypassing the “which” or “where” conundrum altogether.
Another time you would probably favor “where” over “which” is when you’re using an informal contraction in your sentence. Take a sentence quoted in Garner’s Modern American Usage. Mr. Garner says, “In the following example, the contraction ‘I’ve’ might not comfortably fit in the same sentence as ‘in which’—hence ‘where’ is justifiable: ‘I’ve deliberately chosen an example where this unspeakable cluster did not stand out’” (4). I’m not sure what cluster this person is talking about, but that is irrelevant for our current discussion. We’re just pointing out that a formal “which” doesn’t sound right alongside an informal “I’ve.”