Compound Noun’s Plurality

The "base word" (the most "significant" word) adds -s/-es

1. a tennis shoe - tennis shoes
2. a woman-doctor - women-doctors
3. an assistant headmaster - assistant headmasters
4. an assistant secretary of state - assistant secretaries of state
5. a doctor of philosophy - doctors of philosophy
6. sergeant major - sergeants major
7. attorney-general - attorneys-general
8. governor-general - governors-general
9. court martial - courts martial
10. notary public - notaries public
11. commander-in-chief - commanders-in-chief
12. a mother-in-law - mothers-in-law
13. brother-in-law - brothers-in-law
14. father-in-law - fathers-in-law
15. man-of-war - men-of-war
16. toothbrush - toothbrushes
17. a passer-by -passers-by
18. bystander - bystanders
19. maidservant - maidservants
20. step-parent - step-parents
21. lily-of-the valley - lilies-of-the valley

A whole compound noun+s if there is no obvious head noun (both words connected in writing)

1. teaspoonful - teaspoonfuls
2. handful - handfuls
3. bucketful - bucketful
4. cupful - cupful
5. mouthful - mouthfuls
6. toothbrush - toothbrushes
7. haircut - haircuts
8. grasshopper - grasshoppers
9. blackboard - blackboards
10. bedroom - bedrooms

Hyphened nouns with no noun

1. higher-ups
2. also-rans
3. go-betweens
4. has-beens
5. good-for-nothings
6. grown-ups
7. mix-ups
8. take-offs
9. forget-me-nots
10. merry-go-rounds
11. run-ins
12. higher-ups
13. stand-ins

With noun

1. fillers-in
2. hangers-on
3. passers-by
4. makers-up