Q: A colleague (we’re technical writers) insists that one should never use “which” as the object of a preposition (“in which,” “on which,” “by which,” “for which,” “to which,” and so on). Could you comment on this?
A: I’m not sure where your colleague could have gotten the impression that “which” can’t be the object of a preposition. All the phrases you mention (“in which,” “on which,” “by which,” “for which,” “to which,” etc.) are perfectly legitimate. Examples:
Here’s the field in which the plane crashed.
That’s the goal on which he’s fixated.
These are the laws by which we govern ourselves.
Show us the fence over which the prisoner escaped.
We exercised, after which we took a break.
You may be interested in two related items on The Grammarphobia Blog, one about “which” clauses and the other about “which” vs. “that.”
Buy Pat’s books at a local store or Amazon.com.