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 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):
No Principal Word?When there is no obvious principal word, add s (or es) to the end of the compound.
through common usage of both, courts-martial and court-martials are acceptable.)
Compound nouns are nouns that are made up of two or more words. Sometimes, there is a space(s) between the words in a compound noun. Sometimes there is a hyphen(s) between the words, and sometimes the words join to form a single word. For example:
Some compound nouns are hyphenated. For more information, see the lesson Hyphens in Compound Nouns.
BEWARE OF OF
When a compound noun is in the form [word] of [word] (e.g., cup of tea), the first word is always the principal word.
SPOONFULS OR SPOONSFUL?
When a compound noun is in the form [container]ful (e.g., bucketful, cupful, and handful), an s is added to the end to form the plural.
What are nouns?
The different types of nouns
Hyphens in compound nouns
Forming the plurals of abbreviations
Forming plurals (table)