—interrog.adj. asking for choice from a definite set of alternatives (which John do you mean?; say which book you prefer; which way shall we go?).
—rel.adj. being the one just referred to; and this or these (ten years, during which time they admitted nothing; a word of advice, which action is within your power, will set things straight).
—interrog.pron.
1 which person or persons (which of you is responsible?).
2 which thing or things (say which you prefer).
—rel.pron. (poss. of which, whose)
1 which thing or things, usu. introducing a clause not essential for identification (cf. THAT pron. 7) (the house, which is empty, has been damaged).
2 used in place of that after in or that (there is the house in which I was born; that which you have just seen).
Phrases and idioms:
which is which a phrase used when two or more persons or things are difficult to distinguish from each other.
Etymology: OE hwilc f. Gmc
Useful english dictionary. 2012.