As an adjective, key can mean "Of crucial importance" (Oxford). For example: the key facts are the most important facts, or a key worker is an employee whose role is especially vital.

In British English, key in this sense is every bit as common and well-understood as crucial would be. There are some stock phrases where it's especially familiar, but it's not limited to those; you could put it in front of any noun (say, key elephant) and, with the right context, it would make perfect sense ("the matriarch is the key elephant to protect from poachers").

Many years ago, when I was a rookie, someone much more experienced told me that I shouldn't use key as an adjective because "Americans don't understand it". I've been carrying that "fact" with me ever since then – and was just about to impart it to someone else the other day, when I paused to wonder whether it's actually true.

I can obviously see that the meaning is listed in American-centric dictionaries (e.g. Merriam-Webster), but that doesn't help me understand whether it's a common use or an obscure one. I can't find anything about it in any online American style guide but, again, that might mean it's completely uncontroversial or it might mean it's utterly obscure. And the fact the word key has so many different meanings makes it a difficult one just to Google for!

So, can anyone help please?

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    I'm not sure what references I can site (hence comment, not answer), but this born-and-raised American certainly thinks key=crucial is a common, everyday word.
    – Marthaª
    Jun 14, 2015 at 15:15
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    As the other two key players point out, the answer is "sure", but it would be difficult to prove that!
    – Fattie
    Jun 14, 2015 at 15:16
  • @Joe do a corpus search to compare usage frequency. Lots of alternate meanings. Maybe search 'is key'?
    – Mitch
    Jun 14, 2015 at 16:01
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    The Google Ngrams for "key player" / "crucial player" / "key factor" / "crucial factor" for the 'American English' corpus are very much like those for the English corpus. Jun 14, 2015 at 16:07
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    Hi Mitch - for written English, for sure, and I'm always in admiration of folks on here who know how to use that technology. For spoken English, it's a fascinating fact that it is possible the best possible method, extant, in the whole world, is to ask on this web site and get a few me-mates opinions. (Which is conceptually fascinating!)
    – Fattie
    Jun 15, 2015 at 3:25

8 Answers 8


It's a fair question, I suppose – I mean, I believe you were told what you said you were told. However, I'm having trouble imagining anyone on this side of the Atlantic who would be confused by this use of the word key.

As a matter of fact, sportswriters don't usually write for a sophisticated audience, but the Boston Globe wrote about:

Three key plays that won the Super Bowl for Patriots

and the USA Today published a story called:

Gronkowski key part of Patriots' winning Super Bowl

More recently, a sportswriter analyzed Game 1 of the NBA Finals, saying:

Rebounding is a key in any series in basketball, and that figures to be one of the biggest advantages for the Cavaliers. However, it was Golden State who won the battle on the glass, outrebounding Cleveland, 48-45.

This isn't just a sports thing, either. From a recent New York Times interview with a publishing executive:

Every opportunity is a different challenge, and I will quickly assess what the key issues are...

In short, either your friend had an erroneous perception, or else he was pulling your leg.

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    For sure, Americans understand "key" as an adjective meaning "crucial." Perhaps what motivated that person to make that comment about Americans is that the British might use "key" in a wider range of contexts. For example, "key elephant" is understandable but a little odd to my ear. (I am a New Yorker.) Jun 14, 2015 at 15:47
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    @StevenLittman Maybe I overreached with "key elephant" - on returning to the sentence, "understandable but a little odd" is a fair summary of how it reads to me as well. It sounds very much as though my erstwhile colleague was just plain wrong.
    – Morton
    Jun 14, 2015 at 15:52
  • @Morton Since I'm from New York and I've only been in England twice in my life, I wouldn't dream of suggesting I know everything they say over there. Sometimes it sounds like wartime code. (I did have an English nanny for my kids, and I could understand her--summat). But I can speak to what sounds right on the east coast of the US. Jun 14, 2015 at 15:54
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    I generally interpret key X to mean crucial X – except for when I'm ordering key lime pie.
    – J.R.
    Jun 14, 2015 at 15:58
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    @Morton: It's also true that for those of us Americans not from Florida, and thus not familiar with "Key limes" (which are named after the Florida Keys), the name doesn't make any sense unless its explained. Jun 14, 2015 at 16:58

"This is a key idea."

I learned as a child to use the expression above as a compound noun. "Key" was not an adjective, but was part of a compound noun. In recent years, I hear people saying

"This idea is key."

thus treating "key" as an adjective.

"This is the car key."

"Car" is not an adjective here, so you don't say "This key is car." Likewise "key" as I first learned it. Language changes even when we don't like it.

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    this is an outstanding point. Actually could it be that this new usage is rather AmE !??
    – Fattie
    Jun 15, 2015 at 3:29
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    +1 for a valid argument. However, better to buttress every argument with something substantial, canonical, authoritative and easily accessible online. Good Luck.
    – Kris
    Mar 13, 2018 at 7:12

I am from and have always lived in the U.S. and I have always understood "key" as being sometimes synonymous with crucial. Of course, I have also steeped myself in British literature from a young age so that my experience might not be truly representative of the average american. Interestingly enough, however, I ended up on this page because I was typing a paper and used key in that form, and Microsoft Word corrected me, saying that "key" could not be used in this way.

  • That's a comment. Not an answer.
    – Kris
    Mar 13, 2018 at 7:12

I'm always astonished how tenacious the term adjective is for nouns that are part of a compond noun. I know that in English school books everything that is attributed to a noun is called adjective. A very dubious use, as adjective is a word class and no term for describing noun groups (with a noun as main element and all others things as subelements).

In the case of key term/key figure etc we have a special type of nouns, compound nouns. In "an old man" "old" is an adjective. In "raincoat" "rain" is no adjective and English grammar terminology should have a special term for describing the formation of compound nouns. I think it would be reasonable to say "rain" serves as compound element in "raincoat". To say it is an adjective is in my view a kindergarden term.

  • Well... in the abstract I'd agree, but that's what I was trying to get at with the (admittedly dubious) key elephant example. If they were simply accepted specific phrases, in the way that raincoat is, you wouldn't be able just to make one up. Nor do I think we can just write it off as a noun performing the function of an adjective, because its meaning as an adjective is quite different from (although obviously linked to) its meanings as a noun.
    – Morton
    Jun 14, 2015 at 19:02
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    As Michael Hardy points out, this is a word like "fun" that can be used either as an adnominal noun, or as an actual adjective (which can be shown by its ability to be used as a predicate for some people). I agree with the overall sentiment of the post wholeheartedly!
    – herisson
    Jun 14, 2015 at 21:06

Yeah, I've lived in America almost my whole life and we say this all the time. You're good to go.

  • How long is your "whole life"? Jun 15, 2015 at 3:51
  • I'm twenty. Out of the times when I was old enough to be a competent user of language, I spent a year and a half in Germany and about a year in Canada. Jun 15, 2015 at 18:20
  • I've been here three times that long and I agree -- "key" as an adjective is understood by any halfway literate person.
    – Hot Licks
    Feb 8, 2016 at 21:29
  • -1 That's a useful comment. Not an answer.
    – Kris
    Mar 13, 2018 at 7:10
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    @Kris au contraire, it answers the question(s) exactly. "Yeah, " (it's "standard" American English), "I've lived in America almost my whole life and we say this all the time." (evidence). Mar 13, 2018 at 9:26

Based on Ngram, it would appear that, up until about 1985, "key" in this sense was more common in US than in UK literature. It's currently about 40% more popular in UK literature, though.

(I don't know how to embed the actual image, so someone is welcome to edit that in if they wish.)

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It is right, dictionaries label key also as adjective.The Free Dictionary has

  • key decisions, the key element of the thesis; adjective, meaning of crucial importance, significant.

But I think such a label is questionable as so often as to labels of word class in dictionaries. key is no genuine adjective. You can't form comparative forms (key/keyer/keyest) and normally you don't use it predicatively (something is key).

I doubt that you often hear it in this latter use, and if so I would say such use is exceptional and I believe not standard.

"key" if used in adjective position as in "the key figure of the novel" keeps its character as a noun. It is used metaphorically, "the figure as important as the key to a door".

True, we can replace "key" by "crucial", but that does not mean that "key" has changed its word class. Labels of word class in dictionaries are to be considered with a critical eye. It is not rare that they lead astray.


  • Even OALD lists key (something is key) as an adjective (without any restricting remark). I checked BNC to see how often is "...is key". Only four instances. Seems to be en vogue with some speakers at the moment. Nevertheless I see it still as elliptic for "something is of key importance".
    – rogermue
    Jun 15, 2015 at 11:51
  • So, say, "automotive" isn't a "genuine" adjective?
    – Hot Licks
    Feb 13, 2016 at 13:11

The use of 'key' when pedants like me would use 'crucial' or even 'vital' is a blight furthered,in the main,by sportsmen and women on the media,radio,tv etc here in the U.K.It has now become the 'norm' and is used by the gen public on phone-ins,too busy or stupid to use 'crucial'.Notwithstanding its common use it is still grating to me,but then I am a snob.

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    Yeah, you're right. Christopher Wren erred when he failed to call for a "crucialstone" at the top of the aches in his cathedrals.
    – Hot Licks
    Feb 13, 2016 at 13:10
  • Welcome to ELU, John. Wait till you earn the privilege to post comments. These guys here downvote comments posted as answers!
    – Kris
    Mar 13, 2018 at 7:08
  • @HotLicks et al., Why vote someone below ground level?
    – Kris
    Mar 13, 2018 at 7:09

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