What Part of Speech Is Which?

A word doesn’t necessarily belong to a particular part of speech. Depending on how it is used in a sentence, a word can belong to more than one part of speech. For example, word paint can be a noun, a verb, or an adjective:

We need blue paint for the walls. [Noun]

I’ve painted the walls. [Verb]

The paint shop is not yet open. [Adjective]

People find it hard to associate question words such as which with any part of speech, as they think that question words are made just to ask questions and do nothing else in a sentence. But, like paint, they function as quite a few parts of speech.

In this post, we’ll analyze the word which grammatically, looking at the two parts of speech it belongs to: pronoun (two types) and adjective. We’ll also look at few parts of speech it doesn’t belong to, but few erroneously think it does. And all this with plenty of examples.

1. Which in question role

Is which a pronoun?

Yes.

It can function as an interrogative pronoun. An interrogative pronoun is a pronoun used to ask questions, the most common being who, whom, what, which, and whose. Examples:

Which is the best book you read last year?

“Which is the best restaurant in the town?” Sam asked.

Note that which can also act as interrogative pronoun in sentences where it’s not a question word. This has been covered later (under conjunction or subordinating conjunction subcategory) because it doesn’t belong to the direct question category. Examples:

(Comments that go with examples are in square brackets.)

We’ll soon find which is more efficacious vaccine. [Comment: The underlined part is a noun clause.]

I don’t know which to pick. [Not a noun clause]

Which can also be used as relative pronoun where it’s not a question word. This too has been covered later (under relative pronoun subcategory). Example:

The house which was leaning dangerously after the recent flood was razed down by the municipal corporation. [The underlined part is a relative clause.]

If you’re used to seeing only words such as he, she, and you as pronouns, you might struggle to visualize which as a pronoun. But remember every word in a sentence has to be one of the eight parts of speech. Think of which in the above sentences – whether a direct question or not – as referring to a particular thing, a role pronouns perform. And it comes with the added tag of interrogative because it asks questions.

Is which an adjective?

Yes.

It can function as an interrogative adjective. An interrogative adjective, also known as interrogative determiner or just determiner, is also used to ask questions, but, like an adjective, it modifies (or describes) a noun and is placed immediately before the noun. The three interrogative adjectives are which, what, and whose. Examples:

Which ice-cream do you prefer? [Which modifies the noun ice-cream, narrowing down the options to a particular ice-cream, and it comes immediately before the noun.]

“Which restaurant would you recommend for dinner?” Sam asked.

Note that which can also act as interrogative adjective in sentences where it’s not a question word. This has been covered later (under conjunction or subordinating conjunction subcategory) because it doesn’t belong to the direct question category. Examples:

I can’t decide which route you take. [The underlined part is a noun clause.]

I don’t know which route to take. [Not a noun clause]

If you’re used to seeing only words such as hot, tall, and dumb as adjectives, you might struggle to visualize which as an adjective. Think of which in the above sentences – whether a direct question or not – as a word placed immediately before a noun and describing it, just like adjectives do. And it comes with the added tag of interrogative because it asks questions.

2. Which in connecting role

Which does way more than just asking questions: it can connect parts of a sentence. Conjunctions, relative pronouns, and prepositions are the most common grammatical tools to connect one part of a sentence to the other. Let’s examine if which belongs to any of the three.

Is which a conjunction or subordinating conjunction?

(Note that subordinating conjunction is a subcategory of conjunction. If a word is subordinating conjunction, it’ll certainly be conjunction.)

No.

The answer may have come as a surprise to you. When which introduces a dependent clause and joins it to an independent clause, it is commonly treated as a conjunction or subordinating conjunction. But from grammatical perspective, it is not. Let’s dig this up in detail as there is lot of confusion on this point.

What’s a conjunction?

A conjunction is a part of speech that mainly joins clauses but can also join words or phrases. Whereas coordinating conjunctions join grammatically equal elements, subordinating conjunctions introduce dependent clauses and join them to independent clauses. But to be a conjunction, the word can’t play another grammatical role (noun, pronoun, verb, adjective, adverb, etc.) in the clause it introduces. After all, the very same word in the very same sentence can’t be two different parts of speech. Don’t forget that conjunction is one of the eight parts of speech.

There are three types of dependent clauses: adverb clause, relative clause, and noun clause. Here is a summary of whether the connector (we’ll see if it’s a conjunction or not) plays a grammatical role in the clause it introduces.

1. Adverb clause: No connector joining an adverb clause to an independent clause plays a grammatical role in the adverb clause it introduces. Hence, they’re conjunctions. And because they make the clause dependent (or make it subordinate), they can be further subclassified as subordinating conjunctions. Examples: because, since, after, before, while, when, until, etc.
2. Relative clause: All connectors joining a relative clause to an independent clause play a grammatical role in the relative clause they introduce. (They play the grammatical role of pronoun and adverb, which gives these connectors the name relative pronoun and relative adverb.) Hence, they’re not even conjunctions, let alone coordinating or subordinating.
3. Noun clause: The situation is mixed in noun clauses. Some connectors joining a noun clause to an independent clause play a role in the noun clause they introduce and some don’t. Those that don’t play a role (if, whether, and that) are subordinating conjunctions. Others are not.

You can call connectors that are not subordinating conjunctions as subordinators (even though not conjunction, they subordinate the clause they introduce), which would include relative pronouns, relative adverbs, and other connectors (except if, whether, and that) that introduce noun clauses.

With that out of the way, let’s come back to whether which is a conjunction or a subordinating conjunction. Now, which introduces two types of dependent clauses: relative clause and noun clause.

Let’s take noun clause here; relative clause follows.

In case of noun clause, which plays the role of interrogative pronoun or interrogative adjective, just like its role in asking direct questions that we saw earlier. It’s not surprising because noun clauses are often indirect questions. Examples:

We’ll soon find which is more efficacious vaccine. [The noun clause has been underlined. Which is an interrogative pronoun here.]

I can’t decide which route you take. [Which is an interrogative adjective here.]

Since which plays a role (pronoun or adjective) in the clause it introduces, it’s not a conjunction, let alone a subordinating conjunction. You can call it a subordinator or other similar term, which BTW is just an informal term, to denote its role in subordinating the noun clause.

Common error: People mistakenly treat which introducing noun clauses as conjunction or subordinating conjunction. That’s because, in common parlance, the term conjunction is associated with any word that joins two clauses and the term subordinating conjunction with any word that joins a dependent clause to an independent clause. People forget to take into account that it can play another part of speech in the dependent clause, in which case it can’t be a conjunction or subordinating conjunction.

Time for relative clause now.

Is which a relative pronoun?

Yes.

It can function as a relative pronoun. A relative pronoun, if you recall, joins one part of a sentence to another by referring to a noun. It introduces relative clause, which gives more information about the noun referred to. Examples:

The house which was leaning dangerously after the recent flood was razed down by the municipal corporation. [The relative clause has been underlined.]

Coelacanth, which went extinct with the dinosaurs millions of years ago, has been found alive in the Indian Ocean.

Note that in each example which refers to the immediately preceding noun. And because it refers to a noun, it’s a pronoun.

Common error: This error is less common than the one in noun clause, but people sometimes call which a subordinating conjunction, again for the same reason of associating a word that joins a dependent clause to an independent clause with subordinating conjunction. But which, in this role, is a relative pronoun.

Is which a preposition?

No.

Few confuse which with prepositions mainly because both connect two parts of a sentence and both can precede a noun or pronoun. A case in point:

We’ll find about vaccine efficacy after the trials.

We’ll find which vaccine is more effective after the trials.

We’ll find which vaccine to take after the trials.

In all the three sentences, about and which connect two parts of the sentence and are followed by a noun or noun phrase. If about is a preposition, then why isn’t which.

Looking at only the succeeding noun or noun phrase, though, doesn’t present a holistic picture. This does:

We’ll find about vaccine efficacy after the trials.

We’ll find which vaccine is more effective after the trials.

We’ll find which vaccine to take after the trials.

In the second sentence, the noun clause introduced by which works as a unit. In other words, which doesn’t come with just the noun vaccine but lot more, a clause. That’s not what prepositions do.

In the third sentence, if you recall what we covered earlier in the post, which is functioning as an interrogative adjective, implying it modifies noun. That’s not what prepositions do. (For those who are grammatically inclined, the entire phrase which vaccine to take is a noun phrase with an adjective which as a premodifier and an infinitive to take as a postmodifier.)

From the above examples, you’d realize that mere presence of a noun or noun phrase after a word doesn’t mean it’s a preposition. Adjectives can be followed by a noun or noun phrase. So can others be. We’ve to look at the holistic picture.

Bottomline, which is not a preposition. And because it’s not a preposition, it can’t initiate a prepositional phrase.

3. Which in other roles

Is which a verb?

Although rarely, some raise question about which being a verb.

Is it a verb? Not even remotely!

We can trek. We can climb. We can ascend. We can descend.

But can we which?

Which doesn’t do any action. Hence, it’s not a verb.

Another test you can run is to check if which has past, past participle, and present participle forms like verbs do.

Can we’ve the words whiched or whiching, assuming it to be a regular verb?

No. Hence, it’s not a verb.

Few also confuse which with linking verb probably because of its role in linking (or joining) a dependent clause to an independent clause. But it’s not a linking verb. As we saw earlier, which acts as a connector (more specifically, a subordinator) in its role in linking a dependent clause to an independent clause.

Summary

Which mainly functions as a pronoun (with two flavors), but it can sometimes play the role of an adjective:

Which is your favorite story? [Interrogative pronoun]

Which story is your favorite? [Interrogative adjective]

I can’t decide which is my favorite story. [Interrogative pronoun in noun clause. It’s not a conjunction or subordinating conjunction introducing the noun clause.]

I can’t decide which story is my favorite. [Interrogative adjective in noun clause. It’s not a conjunction or subordinating conjunction introducing the noun clause.]

I like stories which have setting in countryside. [Relative pronoun]

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