I don't know exactly how correct I am but I mostly use a rule for things like this that I learned somewhere a long time ago.
If the noun-phrase or compound noun has several heads that are of equal value, when it is obvious that as a whole it refers a single object that cannot be broken down into separate pieces without losing its meaning or is used as a noun to describe an object the S goes at the end of the line, — Sir2u
And frankly I don't even know what I'd have used if it weren't in a linguistic discussion. Maybe I'd have intuitively said "jack-in-the-boxes", too? I don't — Dawnstorm
This thread could in theory lead to a discussion about what grammar is. I come from linguistics, and I've often felt confused about how philosophers use the term grammar. It sometimes feels like philosophers think grammar is the structure of thought, when it's just the structure of language.
"Jack-in-the-box" and where the plural goes is actually a pretty good example. People here keep talking about Jacks and Boxes, but the grammatical structure does suggest you tag the -s onto Jack. — Dawnstorm
The discussion here about "jack-in-the-box" is mostly humorous, but it does show that grammar and thought needn't be the same. — Dawnstorm
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