I think the further you move away from the spatial meaning of "in", the less well "where" works. For example "The house where I live" is OK for "The house in which I live", yet "The year where I was born" is not acceptable for "The year in which I was born". ) However, even when "in" has a spatial meaning, I'd be cautious about saying that "in which" and "where" are always interchangeable.
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KooyeenI think "in which" can't be replaced with "where" if it refers to time.I don't think it's just time though. As I mentioned, I think the same applies whenever the meaning of "in" deviates significantly from the spatial sense (the sense that something is enclosed in something else). For example:
Mr WordyThe original claim that "in which" and "where" are interchangeable is looking more and more dubious.You're right. Judging from the examples in Longman Dictionary of Contemporary English, it seems "where" generally refers to places and situations (in relative clauses).
English 1b3 if the antecedent of 'which' is not a physical location that something can't be insideIt doesn't need to be inside. I think it just needs to be in/on/around/at/... a place.
Mr WordyThat's the place where I'm moving (to). -- Acceptable I guess, but I prefer "That's the place I'm moving to" (more formally you could of course say "That's the place to which I'm moving", but this would sound stilted in conversation).
That's the tree where I'm going to shoot at.