# theorem Isosceles Triangle

I think the sentence 1 is idiomatic:

Sentence 1: The vessel Queen Elisabeth weighed anchor today at 19:00 at the port of Malta.

The word vessel is a noun and "Queen Elisabeth" is the noun used to name the specific vessel.

I wonder whether it would also be possible to follow the same structure in this other context: A mathematics teacher tells his students:

Sentence 2: I am going to teach you today the theorem Isosceles Triangle. The word theorem is a noun and the words "Isosceles Triangle" is the noun specifying the name of the theorem the teacher is going to teach.

With reference to written American English, I would like to know the following. If sentence 2 is not idiomatic and instead "Isosceles Triangle Theorem" should be used, I would like to know why sentence 2 is not idiomatic whereas sentence 1 is idiomatic, considering they both seem to have the same grammar structure.

Thanks

• One difference is that your capitalised "Isosceles Triangle" isn't a proper name. Another thing is that "The Queen Elisabeth vessel" is equally good. But your "theorem Isosceles Triangle" does not sound idiomatic, with "the theorem of isosceles triangles" being more natural. One can't explain why a phrase is not idiomatic: it's a matter of usage, not rationale. Feb 24 at 22:38
• Neither of those are idiomatic. You don't need the appositive in the first one, and vessel names are italicized if at all possible. The Queen Elizabeth ... If you want to specify the class of vessel, then yes, it works like that - the merchant vessel Baltimore or the fishing vessel Beaufort Feb 25 at 14:56
• 'Theorem Pythagoras' sounds almost as outlandish as 'Law Kepler', 'Principle Archimedes' and 'Number Avogadro'. These all have the Saxon genitive in the standard appellation. But I keep thinking there's a counterexample out there somewhere. Feb 25 at 16:41

I am not aware of a theorem called "Isosceles Triangle", but Wikipedia tells me that "Pons Asinorum" is also called "the isosceles triangle theorem".

This indicates that Pons Asinorum is a name for the theorem, but Isosceles Triangle is not. So you could refer to it as "the theorem Pons Asinorum", but not "the theorem Isosceles Triangle".

• Lovely. "The point at which many learners fail." Feb 24 at 22:45
• But even with well-known theorems (I'll not include Saxon genitive forms) such as the Factor Theorem, the Remainder Theorem; the Mean Value Theorem, the Exterior Angle Theorem; the Mid Point Theorem, the Central Limit Theorem, the Cayley-Hamilton Theorem; the Fundamental Theorem Of Algebra ... post-modification is non-idiomatic. Feb 25 at 16:48
• @EdwinAshworth: Indeed. Because "theorem" is part of that name. That was the point I was trying to make. Feb 25 at 21:01
• But if "Pons Asinorum" is also called "the isosceles triangle theorem", "the theorem Pons Asinorum" reads as "the theorem the isosceles triangle theorem". There are extremely few tokens on the internet for "the theorem Pons Asinorum". Probably 'known as' is required. I suspect that the [the] [vessel] Queen Elizabeth], "the play Hamlet", "the novel The Pit and the Pendulum" all work because we're used to merely using the name without the categorising appositive, and the referents are one-offs. Feb 25 at 23:06

Sentence 2 is not idiomatic, and the reason is that "isosceles triangle" is not the name of the theorem but only a modifier; therefore, it is placed before. In the case of "vessel Queen Elizabeth", "Queen Elizabeth" is the name of the vessel and so this group of words can be placed after in a function that is called apposition. The grammatical nature of this function is such that using only one of the elements in the combination "vessel Queen Elizabeth", the sentence is acceptable. Each fulfills the same syntactic function in the modified sentences (here, subject). Also, the two appositives are coreferential (they name the same thing, but that could depend on the particular context), and so there is no difference in what is being said. Appositives of that sort are called full appositives.

It is easy to see that none of the three conditions hold in the case of "theorem isosceles triangle".

(The details can be verified in A Comprehensive Grammar of the English Language, Quirk et al., 1985 edition, § 17.66.)

• With 'The Prime Minister, Rishi Sunak, will have quite a fight on his hands at the next election' either of the two nominals (NPs / DetP + NP) can be omitted without loss of specificity. But with '... vessel Queen Elizabeth' there is certainly loss of specificity if 'Queen Elizabeth' is deleted: it is a specifying appositive. I'm not sure that all appositives don't 'refer to the same thing'. Feb 24 at 23:48
• @EdwinAshworth The first instance I find in Google Books appears to show that there is no loss of identification, but that is very apparently a consequence of context: google.com/… . I agree that in a sentence such as "The ship X takes to the sea tomorrow, but the ship Y only in one month from now.", there is indubitably a loss of identification if X or Y are omitted.
– LPH
Feb 25 at 13:31