Most English nouns form their plurals by adding -s: boy/boys; house/houses.
When the noun is compound, the question sometimes arises as to which word should get the plural ending.
In regard to American usage, the Chicago Manual of Style recommends that writers consult Merriam-Webster’s Collegiate Dictionary for “tricky” compounds like fathers-in-law, courts-martial, and chefs d’oeuvre, adding, “For those not listed, common sense can usually provide the answer.”
I have a lot of respect for the Chicago Manual of Style, but recommending common sense to determine correct usage seems a bit optimistic.
Compound nouns are of three kinds: open, closed, and hyphenated. Not all authorities agree as to which is which or where the plural goes.
Open compounds are written as separate words:
The principal word in the compound takes the plural:
Note: According to the OED, court martials is incorrect; M-W gives courts martial as the first plural, but also accepts court martials.
Most closed compounds form the plural at the end of the word:
toothbrush / toothbrushes
haircut / haircuts
grasshopper / grasshoppers
blackboard / blackboards
bedroom / bedrooms
passerby / passersby
Like open compounds, hyphenated compound nouns pluralize the principal word–if there is one. The principal word will be a noun. Some compounds have more than one noun; others have none. When the compound has two nouns, common sense will have to tell you which one is the principal word.
mother-in-law / mothers-in-law
man-of-war / men-of-war
merry-go-round / merry-go-rounds (no principal word)
forget-me-not / forget-me-nots (no principal word)
stand-in / stand-ins (no principal word)
Some compounds are hyphenated in the Oxford English Dictionary and written as one word in Merriam-Webster:
set-back (OED) setback (M-W)
drop-out (OED) dropout (M-W)
hold-up (OED) holdup (M-W)
half-sister (OED) half sister (M-W)
Some speakers have trouble with nouns that end in -ful, puzzling, for example, between cupfuls and cupsful. This is a case in which common sense should probably advise against consulting M-W.
Although the M-W entries for cupful, handful, and armful list the plurals cupfuls, handfuls, and armfuls first, they give cupsful, handsful, and armsful as alternative spellings. In addition, the spelling handfull is in there as an “also.” My American spellchecker does not countenance any of these alternatives.
Cupsful doesn’t cut it because compound nouns are made up of two or more words that can be used on their own. For example, the words in the compound policeman can be used separately: “The man called for the police.” The element ful in cupful is not a word; it’s a suffix.
Common sense tells me that cupsful is incorrect.