The word key is both an adjective and a noun meaning something is important. But which is the preferred one to use in terms of different territories? For example, should I say

The hint is key.


The hint is the key.

In the first sentence, key means critical, crucial. And in the second sentence, key means the deciding factor for solving a problem.

As an English learner, I'm more interested in AmE and BrE, while usages in other places are also helpful to me. So which one sounds better to the those native speakers? Or are they interchangeable if they both mean something crucially important?

2 Answers 2


As you note, "key" can be either a noun or an adjective.

If you use "key" before a noun, it is usually understood to be an adjective. "Honesty is the key attribute that we are looking for." "Attribute" is a noun. "Key" is modifying this noun.

If "key" is used without a noun to modify, it is usually a noun. "The key to the door is in the drawer." "Key" is not modifying any noun, so it is a noun itself.

Words like this get tricky with predicate adjectives and predicate nominatives. But in general, if there's an article, then it must be a predicate nominate because predicate adjectives don't take articles. So in, "The hint is the key", "key" must be a noun because it has an article. In, "The hint is key", "key" must be an adjective because it doesn't have an article.

But this can be trickier still because we sometimes omit nouns that are repeated. Like, "Honesty is usually the most important factor and competence is the most obvious, but in this case good looks is the key." "Key" here could be a noun, but it could also be an adjective with the noun "factor" omitted. (I suppose in this case the sentence means essentially the same thing either way so it doesn't matter.)

  • In your last example, can I simply say "..., but in this case good looks is key"? Or is it only grammatically correct but sounds weird? Nov 9, 2018 at 20:38
  • "... good looks is key" would be perfectly good. In that case "key" is clearly being used as an adjective because there's no article.
    – Jay
    Nov 9, 2018 at 21:13
  • @Jay "Good looks ARE key" is grammatically correct there. IS key would not be, afaik. Nov 9, 2018 at 21:25
  • @Aethenosity Good point. "Looks" is plural. My bad.
    – Jay
    Nov 9, 2018 at 21:27

Purely in terms of those sentences, the use of key provides different meanings.

The hint is key.

This means that the hint is critical or necessary.

The hint is the key.

This means that the hint is the means of solving or understanding something.

Which you would use depends on what you're trying to express. (The two meanings are not interchangeable.)

Although it would sound strange (and not be how you would normally phrase it), it would even be grammatical to say:

The hint is the key key.

This combines the previous two meanings. It essentially says (through implication):

From among several methods of solving or understanding something, the hint is the one that's necessary.

  • According to Merriam Webster dictionary 2b, key means "an instrumental or deciding factor". In this sense, do the adjective meaning and noun meaning of key interchangeable? In fact, by saying "the hint is the key", I intended to express this kind of meaning. I didn't realize that the meaning could be ambiguous. Nov 9, 2018 at 20:24
  • @zijuexiansheng It's the presence (or absence) of an article that is determining my interpretation in the above. When I read the key, I think of a physical key (a noun) that is used to unlock something. (Which can also have the same sense as what you supplied—but it would be an abstract noun.) When I read just key, I think of an adjective—so, in this case, it's describing something about the hint. Nov 9, 2018 at 20:37
  • Are you saying that the word key, as a noun, will easily lead other people to think about the physical key? In other words, when expressing the meaning of something important, is it better to use the key as an adjective because it leads to less ambiguity? Nov 9, 2018 at 20:43
  • 1
    The difference between these two is so subtle – I can sense it, but it is so hard to describe it. And I don't think Jason is saying that it leads anyone to think about a physical key. "The hint is key" means, "The hint is one helpful way to solve this problem," but, "The hint is the key" means, "You won't be able to solve this problem without the hint." Even that is not exact, though. Like I said, it's a very subtle difference that is quite hard to put into words.
    – J.R.
    Nov 9, 2018 at 20:45
  • @zijuexiansheng Neither noun nor adjective is ambiguous. When I read the in front of key I think of a physical key—but I don't interpret key in the hint is the key as describing something physical because that wouldn't normally make sense. A hint is an idea. So, hearing key in this phrase has me conclude that it's an abstract noun that's being discussed. Reading the key, thinking of a physical key, and knowing that it's talking about the abstract noun (not a real physical key) all happens more or less instantaneously in my mind. Nov 9, 2018 at 20:48

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