adj. & pron.
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). which is which a phrase used when two or more persons or things are difficult to distinguish from each other.
[ OE hwilc f. Gmc ]