A complete guide to the use cases of what and which

Hello, grammar nerds!

Today we're going to discuss one of the most confusing topics in English grammar: the differences between "what" and "which."

These two words have similar meanings but different grammar rules. They function as an interrogative pronoun you can use to ask questions about things. They are also applicable as adjectives, adverbs, and pronouns. 

In some cases, "which" and "what" apply to the same context, but their use cases are usually different.

We'll explain the roles of the words, how to tell them apart, and their correct uses.

The similarities between "what" and "which"

"What" and "which" are both interrogative pronouns. They work to determine the quantity, number, and options for objects, qualities, or places. You cannot use them to refer to a person; the correct word for that purpose is "who." 

• Wrong: What is your best friend?
• Wrong: Which is your best friend?
• Correct: Who is your best friend?

Some particular use cases where you can use them both are:

As subject and object Pronoun

Both "what" and "which" can be used as the subject and object pronoun. In other words, they can function as the verb's subject in a sentence. Examples:

• What do you want?
• Which mobile phone do you want?

The words also function as an object after a preposition. Examples:

• I don't know what he does.
• I don't know which colour to pick.

When the options are already limited

"Which" and "what" are interchangeable when the answer to a question must be chosen from limited options. For examples:

• What bus are you taking to California?
• Which bus are you taking to California?

Both sentences are correct because there are only so many buses operating on that route and going to a particular destination. 

It's important to note that only "what" can replace "which" in some contexts but not vice versa. 

Times or dates

The rule for the previous example also applies here. The number of hours in a day or the days in a month is always limited. So, you can use both "which" and "what" when referring to times and dates. For examples:

What day is the next get-together?

Which day is the next get-together?

The differences between "what" and "which"

Differentiating between "which" and "what" in English grammar can be tricky. However, the main difference lies in the words' functions and meanings, so keep the following points in mind to use them correctly:

Number of possibilities or choices

The main difference between "what" and "which" is the limitless or limited number of possibilities they represent. Using "what" about something implies that there are more options, or the speaker has no idea about the number of choices. 

Applying "which" in the same context means fewer options, and the speaker knows that the answer would be one out of two or several options. 

Examples of what:

• What is your favourite colour?
• What do you want to do during break time?

Examples of which:

• Which flavour of ice cream do you like?
• Which football club did you support last summer?

In sentence structures

Both what and which are used for asking questions. You can use them with nouns and auxiliary verbs in those contexts. 

Usage before nouns:

• Which bike do you want?
• What indoor games are your favourite?

Usage before auxiliary verbs:

• Which is your lucky stone? Ruby or topaz?
• What is the matter with you?

However, "what" is also used with verbs in some cases, which is not the case for "which". Examples:

• What do you do?
• What works for him does not work for me. 

Specified vs. unspecified

The word "which" refers to a specific noun or pronoun. 

• For example, "Which book are you reading?" (Meaning some specific books that the speaker already knows about.)

On the other hand, what implies something unspecified.

• For example, "What do you want for your birthday?" (Meaning something unspecified or an unknown quantity or object the speaker does not know about.)

The punctuation use

"Which" can always be preceded by a comma, while "what" cannot. For example: 

• I said something hurtful, which I did not mean to.
• I'm not sure what you mean.

Already referenced to or not

The word "which" refers to things, animals, people, or ideas that have already been mentioned in sentences or conversations. 

• For example: The car, which I drove last night, was red.

Using the word "what" in these contexts implies that the speaker is unaware of the subject. 

• For example: What car did you drive last night?

Instead of referring back like "which" does, "what" asks for more information about something specific. For instance:

• What games do you want to play?
• Which games do you want to play?

The first question asks you to talk about all the games you like to play. It also suggests that the person asking the question does not know anything about your choices.

The second question is more specific, asking you about something from a limited pool of options. It also indicates that the speaker knows the answer would be from several options. 

FAQs about what and which

What is the difference between what and which?

"What" and "which" have particular use cases. The key difference is their use in indicating the number of choices or options in a sentence. "Which" is used when there are a limited number of choices, and the word "what" refers to an unlimited number of options. 

What is the rule for using "which?"

You can use "which" in contexts of limited answers or when the answer involves a limited choice pool. It could also be something that the speaker already knows. 

What is "which" in grammar?

"Which" is an interrogative pronoun in the English language. It also functions as an interrogative determiner and relative pronoun. Examples:

• Which is the easiest chapter? (interrogative pronoun)
• Which chapter is the easiest? (interrogative adjective)
• Which black pencil do you want? (interrogative determiner)
• The house, which Max wanted to buy last summer, is haunted. (relative determiner)
• He confessed to cheating on his wife, which surprised me. (relative pronoun)