Why "in which" comes here?

Only which (not that) can come after a preposition. The preposition is in.

The prisoners are incarcerated in that place.

So in must be used with which.

CJ

You could use a good spellchecker, Bpk. You got a couple of the hard ones right, but missed five others.

Rgdz, - A.

What I asked was this that if we can replace the "in which" with others like: which, that, in that... The preposition comes with "which" not with the sentece.

A penitentiary is a place in which/which/that/in that perisoners are incarcerated as punishment for crimes they have committed.

Which of the aboves could come with the sent

A dog house is a thing in which dogs are kept.
A dog house is a thing which dogs are kept. Some people would say "which dogs are kept in."

A dog house is a thing in that dogs are kept. "That" doesn't work following "in," as CJ said.

A dog house is a thing that dogs are kept. You could say, "a thing that dogs are kept in."

Thanks, that's Helpful.

Beating the dead horse:

It keeps nagging me that we've said "that can't follow a preposition," or "that can't be used with in."

"That" is a very busy word, and is used in many ways, some of which, I confess, confuse me.

We would commonly and correctly say, "Your answer is incorrect in that you have failed to capita

Avangi"Your answer is incorrect in that you have failed to capitalize the first word of the sentence."

Yes. in that with the meaning because is possible because thatis a complementizer here which needs a full clause after it -- not a relative pronoun that refers back to some antecedent noun. I often sacrifice completeness by not mentioni