forming plurals of compound nouns

The Quick Answer
When 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 Word

When 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.)

See Also

What are nouns? The different types of nouns Hyphens in compound nouns Forming the plurals of abbreviations Forming plurals Forming plurals (table)