forming plurals of compound nouns
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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 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. Therefore, 
      through common usage of both, courts-martial and court-martials are acceptable.)
 
COMPOUND NOUNS? 

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:

  • Jack-in-the-box
  • Knight Templar
  • Lieutenant general 
  • Court-martial
  • Forget-me-not
  • Toothbrush
  • Water bottle
  • Ink-well
  • Board of Education

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.

  • 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, and 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.
 

See also:

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







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