Plural noun agreement?

The tops of both arms.

Justification for boss: two arms, two tops.

Thanks so much. That was my opinion, but I could not find this specifically addressed in various grammar searches. "Plural noun agreement" brought up lots of info but did not address this. Is there a more correct term?

It all depends on what you want to say. When asking it is preferable to show the whole sentence. For example your sentence could have been "Draw the line that joins the tops of both arms", but you could as well had been referring to something that related individually to "the top of each arm". You have to find the way that expresses it with greater clarity. Grammatical correctness does not guarantee clarity of meaning.

We refer to the "upper arm" sometimes.

(which might give extra problems if you want to refer to both!)

Nowadays the word "deer" may be pluralized, I believe. But when I was in school, it was one of those words that was not to be pluralized (like fish).

Anyway, if one says, "on the head of the deer," or "on the heads of the deer" it is only the word head that signifies whether it is one or more deer. This might add weight to the concern about "tops" or "top"?

It still is. The people who write or say "deers" are making an error. Likewise for sheep, moose, swine, fish including varieties (e.g. a hundred cod, twenty salmon).

Confusingly, there are some creatures which commonly have plurals ending in -s except when they are spoken of by hunters e.g. I saw a pair of lions but a hunter might say he shot "a brace of lion". There is a US colloquial expression "loaded for bear" which means "ready and eager to deal with something that is going to be difficult", it comes from the fact that a hunter who hopes to successfully shoot a bear would need to load his weapon with heavy ammunition.



That is correct. However, context and use of vocabulary can be used to help.

"I shot the deer which stood near the oak tree" is ambiguous, but "I shot the the large buck which stood near the oak tree." is not.



The plural of "arm" is not "arm", so the problem will not arise in the mind of a sufficiently skilled speaker.

Certainly adding an 's' to 'deers' and 'elks' is not nearly as common as 'deer' and 'elk'. But to suggest that a minority usage would be in error would set much of BrE firmly in the "wrong" department.

All right, if you like, saying "deers" or "elks" is non-standard, and people (like the OP and many of the people who post on here) learning ESOL should steer clear of non-standard usages until they are sufficiently fluent to make the appropriate judgement about when to use them.