What is the difference between "in which" and plain "which"? Can they be used interchangeably? For instance what is the difference between these two sentences:
The party in which I attended was fun.
The party which I attended was fun.
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A classic example of in which not being replaceable with which may be seen in the film In Which We Serve.
Of course, to be strictly correct, it is the title of the film in which the example can be seen, not the film itself.
More broadly, if Thing 1 can be verb + in Thing 2, then Thing 2 is the thing in which Thing 1 is verbed.
Paddington Bear got lost in the Early Stamp Section of the Royal Postal Museum. The Early Stamp Section was subsequently much remarked upon for being the section of the museum in which Paddington Bear got lost.
There are many ways to phrase a thought. However, in which and similar expressions make it possible for the writer to put their main idea at the place in the sentence that gives it the greatest impact - the point at which the speaker’s emphasis most naturally falls.
As a general answer, I don't feel like they can be used completely interchangeably, but there may be sentences where they can be swapped with little to no semantic change and still remain grammatically correct.
M. C. asked a question in which the difference between 'in which' and plain 'which' was raised.
M. C. asked a question which raised the difference between 'in which' and plain 'which'.
Both of these sentences are valid and mean the same thing. But the sentence had to adjust to accommodate dropping the 'in'. If I knew your pronouns, the first version could be modified to a more active tense for the prepositional phrase which would require a more substantial change to drop the 'in'.
I do feel more examples could help qualify exactly how to describe the difference.
See also How to correctly apply "in which", "of which", "at which", "to which", etc?, in which the matter is further discussed, though I don't feel it makes this a duplicate because that was closed as being off topic, possibly for being too broad.