Today I found myself in a situation that I asked "Which class" to a friend who said that "we are going to the class". Then one of my friends said that I should ask "what class" and another one said both (what class and which class) are correct. Who is right?

This issue I meet often when I'm talking about things, such as:

What day/ which day?

What floor / which floor?

what teacher / which teacher?

What page / which page?

What place / which place?

What line (bus) / Which line?

What /which sentence (do you agree with)?

2 Answers 2


This is actually a rather simple answer although it's rarely explained in simple terms:

What seeks the answer from an undefined set while which asks you to choose among possible options. (Additionally, you could ask which when discussing people.)

For instance, to use your first example, let's say that I was making an appointment with a friend:

"What day are we meeting?"
"I'm available on Tuesdays and Thursdays."
"Which one works best for you?"
  • I should emphasize that the set of possible options can actually be quite large. For instance, when your friend asks what/which class you're taking, either usage is acceptable if we're referring to many possible options. If your friend asked for the class that you were taking to fulfill a requirement in the chemistry department, then 'which' is more appropriate for a more limited set of options. Commented Nov 28, 2016 at 20:11
  • 2
    A rather nice, concise, and cogent explanation. I think I'll steal it :)
    – Andrew
    Commented Nov 28, 2016 at 20:23
  • Thank you for you answer. But still I do not find the way to apply your things in the reality because all of the examples that I put on the post are a matter of options. (i.e. always when I ask about a day, floor, teacher, and page etc. it's necessary that I have options such as: day from other days, floor from other floors, teacher from other teachers and pages from other pages etc.). So in the end of the day, I don't understand how I can use your answer in fact. Commented Nov 28, 2016 at 20:46
  • 1
    The keywords here is set. If I asked you, "What kind of art do you like?" then I would be asking a very open-ended question. On the other hand, I could ask you "Which kind of art do you like?" In this instance, it would be a strange question unless your options were limited (a defined set of options). In fact, there are many instances where the words are interchangeable but we're talking about a tendency in usage. Commented Nov 29, 2016 at 16:25

My English is not very good but I understood that Which should be used when we are talking about possible options . @ What : undefined .

@ what day? : can be any from Monday to Sunday @ which day ? : when we know we can only choose between certain days

@ what floor? : could be any floor in that building @ which floor?: when you know that that Hospital has 5 floors and you are looking after Pediatric Department which could be either floor 1 or floor 2 ;

@ I hope it makes sense . Is what I understood from those previous explanations .

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