Longman Dictionary of Contemporary English homepage

Language: Old English
Origin: hwilc

which

 determiner, pronoun
     
which S1 W1
1 used to ask or talk about one or more members of a group of people or things, when you are uncertain about it or about them:
Which book are you looking for?
Which are the most important crops?
Miranda was sure it was one of them, but was not sure which.
which of
I don't know which of us was the more scared.
2 used after a noun to show what thing or things you mean:
Did you see the letter which came today?
Now they were driving by the houses which Andy had described.
! In informal and spoken English, it is more usual to use that: This is the one that I wanted.
3 used, after a comma in writing, to add more information about the thing, situation, or event you have just mentioned:
The house, which was completed in 1856, was famous for its huge marble staircase.
One of the boys kept laughing, which annoyed Jane intensely.
He was educated at the local grammar school, after which he went on to Cambridge.
She may have missed the train, in which case (=if this happens) she won't arrive for another hour.
4

don't know/can't tell etc which is which

if you do not know which is which, you cannot see the difference between two very similar people or things:
The twins are so alike I can never tell which is which.
Definition from the Longman Dictionary of Contemporary English
Advanced Learner's Dictionary.
 
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