Compound descriptive nouns, or whatever you want to call them. It's 11pm, I've nothing better to think about.

Court martial. Sergeant Major. Father-in-Law.

Plural is easy. Just add an 's', as the noun part is easy to identify, the second term is an adjective.

Courts martial. Sergeants Major. Fathers-in-law.

What's throwing me right now is the posessive. Where to put the apostrophe 's'? Whilst placing it in the same location as the plural 's',  makes logical sense, it certainly doesn't sound right.

"It's in the Sergeant's Major HMMWV"

vs

"It's in the Sergeant Major's HMMWV" which sounds more natural.

How about plural posessives?

"They're in the Sergeants Major's HMMWVs"

vs

"They're in the Sergeant Majors' HMMWVs" (Or if you're old-fashioned, Sergeant Majors's)

I'm leaning towards putting the apostrophe 's' on the adjective, but I can't think of a logical reason why that should be the case.

Any ideas?

NTM

Sergeant Major's, because that's the way it is. It may not make that much sense, but that's the English language for you.

The plural posessive is Sergeant Majors'.

"It's in the Sergeant Major's HMMWV" <--- that one, otherwise it sounds as though there are more than one Sergeant Major (Sergeants Major vs. Sergeant's Major, same pronunciation)