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.)
Compound nouns are nouns made up of two or more words.
Sometimes, there are spaces between the words in compound nouns. Sometimes, there are hyphen between the words, and, sometimes, the words merge to form a single word. For example:
Spaces between the words
- Knight Templar
- Lieutenant general
- Water bottle
When a compound noun is in the form [word] of [word] (e.g., cup of tea), the first word is always the principal word.
- I sold them 4 cup of teas, but they only drank one. (should be cups of tea)
SPOONFULS OR SPOONSFUL?
When a compound noun is in the form [container]ful (e.g., bucketful, cupful, handful), an s is added to the end to form the plural.
- There were 3 spoonsful of honey left in the jar. (should be spoonfuls)
- Please sprinkle two handfuls of corn on the porch for the chickens.