forming plurals of compound nouns
The Quick AnswerWhen a compound noun is in the form brother-in-law (i.e., with hyphens) or made up of two or more words (e.g., Knight Templar), form the plural by adding s to the principal word in the compound. If there is no principal word or the compound noun is a single word, follow the usual rules (which will usually just mean adding s to the end of the word).
Pluralize the Principal WordWhen the words in a compound noun are joined by hyphens (e.g., Forget-me-not, brother-in-law) or when the words are separated by spaces (e.g., dry dock, Knight Templar, Lord Lieutenant), the compound noun will usually form its plural by adding s to the principal word in the compound. When the compound noun is a single word (e.g., banknote, letterhead), the plural is formed using the usual rules for forming a plural, which, more often than not, is adding s to the end.
Examples (principal word in bold):
- He now has two mothers-in-law. (plural of mother-in-law)
- They were visited by the Knights Templar. (plural of Knight Templar)
- It was a sight to see four lieutenant generals fight it out at the table. (plural of lieutenant general)
- Jerry had attended over a dozen courts-martial. (plural of court-martial)
(Also, see the third example below.)
No Principal Word?When there is no obvious principal word, add s (or es) to the end of the compound.
- Forget-me-nots make a wonderful present. (plural of forget-me-not)
- Pack two toothbrushes. (plural of toothbrush)
- Jerry had attended over a dozen court-martials. (There is ambiguity about the principal word in court-martial. Both courts-martial and court-martials are acceptable due to common usage of both terms.)