Discussion Groups / English Usage / January 2010

I don't think I ever learned a special grammatical term for "in which"
or "by which".  Each of them is a prepositional phrase with a relative
pronoun as the object of the preposition.  There is a Wikipedia
article, in this list:
http://preview.tinyurl.com/yfmkjtg , that calls the construction an
"adpositional complement".  You might try searching for that, but it's
not a term I know, and a brief search that I made didn't seem very
productive.

It's easier to talk about a whole sentence than to explain a short
phrase.  In "This is the house in which I live," the relative pronoun
"which" is used to connect two statements: "This is the house," and "I
live in the house."

The relative pronoun in the second statement stands for "the house"
(you could think of it as an abbreviation for "the house"), and is
also the object of the preposition "in" in the second statement, so
that the two statements become a single sentence.