Forming Plurals of Compound Nouns

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Pluralizing the Different Types of Compound Noun

This infographic summarizes how to pluralize a compound noun:

how to form the plural of a compound noun

(Rule 1) Forming the Plurals of Hyphenated Compound Nouns

When a compound noun is hyphenated, add "s" to the principal word. (In these examples, the principal words are in bold.)
Compound NounPlural
brother-in-lawbrothers-in-law
passer-bypassers-by
water-bottlewater-bottles
Beware the Exception! There is one notable exception to this rule. If the compound noun consists of a verb and preposition (see Rule 8), add "s" to the end.
Compound NounPlural
love-inlove-ins
Read more about compound noun with hyphens.

(Rule 2) Forming the Plurals of Multi-Word Compound Nouns

When a compound noun consists of two or more words, add "s" to the principal word. (In these examples, the principal words are in bold.)
Compound NounPlural
Knight TemplarKnights Templar
bus stopbus stops
full moonfull moons

(Rule 3) Forming the Plurals of Compound Nouns without a Principal Noun

If there is no principal word, follow the usual rules for forming a plural, which usually means adding "s" to the end of the word.
Compound NounPlural
forget-me-notforget-me-nots
court-martialcourt-martials*
* "Courts martial" is also an acceptable plural of "court martial." This version is used by those who deem "court" to be the principal word.

(Rule 4) Forming the Plurals of Single-Word Compound Nouns

When the compound noun is a single word, follow the usual rules for forming a plural (i.e., pluralize the ending).
Compound NounPlural
breakwaterbreakwaters
rainforestrainforests
seagrassseagrasses

(Rule 5) Forming the Plurals of Terms in the Form [Word] "of" [Word]

With a term in the form [word] of [word] (e.g., "cup of tea"), the first word is always the principal word. Form the plural by pluralizing the first word.
word
+
"of"
+
word

Compound NounPlural
cup of teacups of tea
man of warmen of war
right of passagerights of passage

(Rule 6) Forming the Plurals of Compound Nouns Ending "Ful"

When a compound noun in the form [container]-"ful", by far the safest option is to add an "s" to the end to form the plural. However, it might be acceptable to add an "s" to the first half of the compound.
noun
+
"ful"

Compound NounPlural
bucketfulbucketfuls
cupfulcupfuls
handfulhandfuls
mouthfulmouthfuls
spoonfulspoonfuls
truckfultruckfuls
Many native English speakers feel that "bucketsful" sounds more natural than "bucketfuls." If you are a native English speaker, you can follow whichever version sounds more natural to you. However, if you use two or more of these terms in the same document, you should strive for consistency.

Unsure? If you're unsure, add "s" to the end. Sorted.

More technical help? If you're still unsure which version to choose (e.g., "mouthsful" or "mouthfuls"), use Google's Ngram viewer (which searches millions of books for the terms). This tool tells you - in less than a second - how the terms have been used over the last two centuries. Here are some examples:

(Rule 7) Forming the Plurals of Compound Nouns with a Noun and a Preposition

When a compound noun consists of a noun and a preposition, pluralize the noun.
noun
+
preposition

In these examples, the nouns are in bold.
Compound NounPlural
passer-bypassers-by
hanger-onhangers-on

(Rule 8) Forming the Plurals of Compound Nouns with a Verb and a Preposition

When a compound noun consists of a verb and a preposition, add an "s" to the end.
verb
+
preposition

In these examples, the verbs are in bold.
Compound NounPlural
check-incheck-ins
build-upbuild-ups
take-outtake-outs
love-inlove-ins
Interactive Exercise
Here are three randomly selected questions from a larger exercise, which can be edited, printed to create an exercise worksheet, or sent via email to friends or students.

See Also

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