Plural Forms of Compound Nouns:
- Generally, the plurals of compounds are formed by pluralizing the main element of the compound: fathers-in-law.
- Some compounds have two plural forms. Here are two additional examples (the first form is preferred): courts-martial, court-martials.
- When a hyphenated compound does not contain a noun, the final element can be pluralized: know-it-alls.
sergeant major sergeants major
assistant headmaster assistant headmasters