There's an expression: "to know what's what", as in: "He's been around for a long time - he knows what's what." (ref: Cambridge Dictionary).

But recently I started wondering: isn't it an embedded question? If so, shouldn't it actually be: "He's been around for a long time - he knows what what is."?

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    "what's what" is idiomatic, just like "which is which".
    – Centaurus
    Apr 9, 2022 at 21:53
  • To be honest "idiomatic" mostly means "everyone's always been using it that way and that way only". Doesn't mean everyone hasn't always been wrong. (And yes, I know in natural languages if enough people have been making a mistake for long enough that mistake may eventually become the norm and therefore the correct way but that doesn't mean it wasn't a mistake initially).
    – NPS
    Apr 9, 2022 at 22:05
  • Why do you presuppose it might be a mistake? It's precisely because it is an idiom that it has inversion. "What's what" means the true or real situation, whereas "What what is" seeks information about the referent of "what", which is of course meaningless.
    – BillJ
    Apr 11, 2022 at 8:08
  • 'He knows who is there/what is appropriate ....' May 10, 2022 at 13:47
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    Compare "knows who is who", "knows which is which", etc.
    – Stuart F
    May 11, 2022 at 11:11

3 Answers 3


To clarify syntax, I will mark subjects with s in this answer. I will follow the usual convention of marking unacceptable sentences with a preceding *.

The word order in "He knows whats is what" is correct and does not have any subject-auxiliary inversion: the subject (the first what) comes before the auxiliary verb is, which comes before the predicative complement (the second what).

Syntax of interrogative clauses with multiple wh-words

"He knows whats is what" contains two wh-words in the same clause: one as the subject, and one as the predicative complement (as MarcInManhattan rightly notes).

The use of more than one wh-word in the same clause like this is not common in formal language, but when it occurs, more than one wh-word can't be put at the start of the clause, so any additional wh-word(s) beyond the first stay in place rather than undergoing wh-fronting. In this case, the predicative complement what stays in place (or "in situ"). This is normal English grammar for this context; it is not exceptional.

Per Glottopedia's article on wh-in-situ:

in languages with overt movement of one wh-element (like English), the other wh-elements stay in situ.

In the case of whats is what, since the clause has a wh-word as its subject, no subject-auxiliary inversion is possible, whether the clause is embedded or not (compare Whos did it? I don't know whos did it, not *Did whos it? or *I don't know did whos it?).

We do see subject-auxiliary inversion (with do-support if needed) in a non-embedded interogative with multiple wh-words where the first wh-word is not the subject: "What did hes put where?" And when embedded, we would have wh-fronting without subject-auxiliary inversion as "I don't remember what hes put where."

A priori, we might suppose that it should be syntactically possible to front the predicative complement, and leave the subject what in situ, which would result in it inverting with the verb. That would create "What is what?s" and when embedded, "What whats is". But I don't think this actually sounds natural to any speakers, so I don't think that is the structure that is actually used in the expression "know what's what".

In practice, there seem to be restrictions in interrogative clauses with multiple wh-words on which ones can be fronted.

These restrictions seem to be called "superiority effects"; here is a quote from literature studying that topic in a different context:

for speakers that do allow wh-extraction of indirect objects, the predicted contrast does appear to hold, as shown in (85a-b).

(85) a. ?? Whomi did John give ti what?
b.* Whati did John give whom ti?

(Symmetry in Syntax: Merge, Move, and Labels, by Barbara Citko)

Other analogous examples

Other examples of the same type: "I don't know whos did what" and "I don't know whats goes where" (because the subjects are who and what respectively, the additional wh-words what and where remain unfronted).

Note that this word order is much more acceptable than "*"I don't know what whos did" or *"I don't know where whats goes."


The word order is correct.

Other people may parse this differently, but one way is to consider "what's what" as a nominal clause (functioning as the object of "know"). The first "what" serves as the nominal clause's introductory word (subordinator). It also functions as the clause's subject, so the rest of the clause follows in regular order: the simple predicate ("is") comes next, followed by the predicate nominative (the second "what").

It is a bit unusual for both a subject and a predicate nominative to be wh-words, but in this case it is idiomatic.

By the way, it is also possible to write "he knows what what is", but in that case the first "what" would be the predicate nominative and the second "what" would be the subject. The meaning would therefore be slightly different and wouldn't seem to make much sense (although it might in some contexts).

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    Embedded questions like He knows what's what, I wonder what he meant, Why he did it is a big mystery are one of the four complement clause types in English. It's one of the two tensed complement types, along with that complements (the two untensed complement types are infinitives and gerunds). As you can see, such complement clauses can be subject or object, but only with certain verbs, which vary wildly. The embedded question type is distinguished by not performing subject-auxiliary inversion, the way real questions do. Apr 9, 2022 at 22:23
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    No Marc: I agree partly with JL: "what's what" is indeed a subordinate interrogative clause (embedded question) functioning as complement of "know". The meaning is "I know the answer to the question 'What's what?'". I disagree that it's an object: objects are virtually always noun phrases, but never clauses.
    – BillJ
    Apr 10, 2022 at 10:21
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    We could have cut to the chase much earlier had you indicated that you interpret this sentence's subject & PC differently! As I mentioned in my answer, the first "what" is the subject, so "what is what" is uninverted. Cf: "I know who steals the cookies." The wh-word in first position is the subject. The item in third position can't be the subject: *"I know whom steal the cookies." It's a bit more subtle with the copula, but if you think about it I think you'll see that it's correct. Apr 12, 2022 at 20:54
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    Quote from an authoritative textbook: in the main clause "What are they?" "what" is PC, a non-subject. It is fronted and is accompanied by obligatory subj-aux inversion. The same applies if it is (exceptionally) used as a subordinate interrogative.
    – BillJ
    Apr 15, 2022 at 13:03
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    I don't see the relevance of that example. The simple fact is that in "What are they?", the subject is "they" and "what" is PC, thus that clause has inversion. Are you disputing this?
    – BillJ
    Apr 17, 2022 at 9:13

Yes, because in short, you know, what is what. 'know what is what' is the correct order of words. No order of words would make any sense.

For example, what^2 would be what you know is. what^1 is regular usage.

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    Is there an 'other' missing in the last sentence? May 10, 2022 at 17:27
  • @KillingTime I do not understand your question.
    – bmp
    May 10, 2022 at 17:29
  • 1
    Try reading your first paragraph again. The final sentence contradicts the previous one. May 10, 2022 at 17:50

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