Compound nouns are of three kinds: open, hyphenated, and closed.
As the names imply, “open compounds” are written as separate words, “hyphenated compounds” are written with one or more hyphens, and “closed compounds” are written as a single word.
Many compounds begin as open, progress to hyphenated, and finish as closed. Because of the modern preference to avoid hyphenating words as much as possible, newly created compounds tend to develop closed forms earlier than they might have in the past. Some compounds written as one word in US usage are hyphenated in British usage.
Compound nouns are formed by combining different parts of speech. This list of ten is not exhaustive.
1. noun + noun
2. noun + preposition/adverb
passerby (Br. passer-by)
3. noun + adjective
4. noun + verb
5. adjective + noun
6. adjective + verb
7. preposition/adverb + noun
8. verb + noun
9. verb + preposition/adverb
10. word + preposition + word
Most compound nouns form their plurals like any other noun: by adding an s to the end of the word: wheeler-dealers, washing machines, onlookers.
A few, like mother-in-law and hole in one do not place the s at the end, but on the most significant word: mothers-in-law, holes in one.
Some compounds of French origin in which the adjective stands last have more than one acceptable plural (depending upon the dictionary):
attorney generals or attorneys general
court martials or courts martial
film noirs, films noir, or films noirs
runner-ups or runners-up
Because there are no hard and fast rules regarding the writing of compound nouns, stylebooks advise writers to consult a dictionary when in doubt.