"What" asks about something from an UNLIMITED or unknown set of options; "which" asks when there is a LIMITED, DEFINED set of options to choose from.
WHAT: interrogative pronoun for questions with an OPEN or UNLIMITED universe of possible answers. The speaker does not limit the options in advance. "What color is your car?" = any color in the spectrum is possible. "What time is it?" = any time of day. "What would you like to drink?" = the beverage menu is wide and unspecified. WHAT is used when the answer is not restricted to a defined set of known options. Also used to ask about GENERAL CATEGORIES, PROFESSIONS, or NATURE: "What do you do (for work)?" = what is your job / what is your profession.
What approach does the company currently use for managing cross-functional dependencies between product and engineering?
What specific metrics should the dashboard display to give the executive team the most relevant real-time overview of performance?
What did the client say during the feedback session that made you feel the relationship might be at risk?
WHICH: interrogative pronoun for questions within a LIMITED, KNOWN set of options. "Which color do you prefer, blue or green?" = the options are defined (blue or green). "Which report should I read first?" = the reports already exist and the speaker knows the set. "Which flight are you taking?" = available flights are a limited set. WHICH implies that the speaker and/or listener already know what the possible options are — you choose WITHIN that set. The phrase "which one?" is common when the set has already been presented: "Here are three options. Which one do you prefer?" WHICH can stand alone or precede a noun: "Which?" / "Which option?"
We have two potential vendors for this project. Which one offers the better combination of price, delivery timeline, and after-sales support?
Which of the three marketing strategies tested during the pilot generated the highest conversion rate among the target demographic?
Which candidate do you think is the strongest fit for the chief operating officer position based on today's interviews?
Common mistake
Most common error: using WHICH when options are open, or using WHAT when options are a limited set. "What do you prefer, coffee or tea?" = grammatically acceptable but WHICH would be more precise because the options are defined (coffee or tea = set of 2). "Which is the best city in the world?" = WHAT would be more appropriate because "best city in the world" has no pre-established set of options. In practice: native English often uses both in informal contexts without a rigid distinction. The difference is more marked in formal written English and in questions about VERY clearly defined sets (one of two, one of these three).
Trick to remember it
The SET TEST: do the possible options form a LIMITED AND KNOWN GROUP for the speaker? YES = WHICH. NO (options are open or unknown) = WHAT. Practical test: add "of the [set]" after. "Which of the three candidates?" = WHICH because there is a defined set of 3. "What candidate would you recommend?" = WHAT because there is no pre-established set. Signal of limited set: when the group of options has already been mentioned in the question or context ("here are four options, which one...?") = always WHICH. Signal of open set: when the question does not presuppose a specific set of options = WHAT.
Quick rule
What = open choice (unlimited or unknown options). Which = choice within a limited, known set. "What is your favorite color?" (unlimited). "Which do you prefer, red or blue?" (limited to 2 options).
Did you know?
The WHAT vs WHICH distinction in English reflects a difference logicians call "restricted vs open domain reference." WHAT operates on an open domain (any possible answer). WHICH operates on a closed domain (an already-defined subset). Interestingly, English is one of the few European languages with two distinct interrogative words for this difference. Spanish uses "que" and "cual" similarly but with slightly different rules: "cual es tu color favorito?" (cual = which, more formal) vs "que color te gusta?" The rules of cual/que in Spanish are not identical to those of which/what in English, which means direct transfer sometimes leads to errors. In particular, Spanish more freely accepts "cual" where English requires "what."
Exercise 1 · Pick the right one
_____ project management methodology does your team currently use, and how long have you been applying it?
Exercise 2 · Pick the right one
We have shortlisted three vendors for the IT infrastructure project. _____ one offers the strongest post-implementation support and SLA guarantees?
Exercise 3 · Form the word
What = open choice (unlimited or unknown options). Which = choice within a limited, known set. "What is your favorite color?" (unlimited). "Which do you prefer, red or blue?" (limited to 2 options).
Most common error: using WHICH when options are open, or using WHAT when options are a limited set. "What do you prefer, coffee or tea?" = grammatically acceptable but WHICH would be more precise because the options are defined (coffee or tea = set of 2). "Which is the best city in the world?" = WHAT would be more appropriate because "best city in the world" has no pre-established set of options. In practice: native English often uses both in informal contexts without a rigid distinction. The difference is more marked in formal written English and in questions about VERY clearly defined sets (one of two, one of these three).
The SET TEST: do the possible options form a LIMITED AND KNOWN GROUP for the speaker? YES = WHICH. NO (options are open or unknown) = WHAT. Practical test: add "of the [set]" after. "Which of the three candidates?" = WHICH because there is a defined set of 3. "What candidate would you recommend?" = WHAT because there is no pre-established set. Signal of limited set: when the group of options has already been mentioned in the question or context ("here are four options, which one...?") = always WHICH. Signal of open set: when the question does not presuppose a specific set of options = WHAT.
Bryan López
Founder of SpeakUP Academy