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Plural Forms of Compound Nouns

In general we make the plural of a compound noun by adding -s to the "base word" (the most "significant" word). Look at these examples:

singular plural
a tennis shoe three tennis shoes
one assistant headmaster five assistant headmasters
the sergeant major some sergeants major
a mother-in-law two mothers-in-law
an assistant secretary of state three assistant secretaries of state
my toothbrush our toothbrushes
a woman-doctor four women-doctors
a doctor of philosophy two doctors of philosophy
a passerby, a passer-by two passersby, two passers-by

Note that there is some variation with words like spoonful or truckful. The old style was to say spoonsful or trucksful for the plural. Today it is more usual to say spoonfuls or truckfuls. Both the old style (spoonsful) and the new style (spoonfuls) are normally acceptable, but you should be consistent in your choice. Here are some examples:

  old style plural (very formal) new style plural
teaspoonful 3 teaspoonsful of sugar 3 teaspoonfuls of sugar
truckful 5 trucksful of sand 5 truckfuls of sand
bucketful 2 bucketsful of water 2 bucketfuls of water
cupful 4 cupsful of rice 4 cupfuls of rice

Some compound nouns have no obvious base word and you may need to consult a dictionary to find the plural:

Note that with compound nouns made of [noun + noun] the first noun is like an adjective and therefore does not usually take an -s. A tree that has apples has many apples, but we say an apple tree, not apples tree; matchbox not matchesbox; toothbrush not teethbrush.

With compound nouns made of [noun + noun] the second noun takes an -s for plural. The first noun acts like an adjective and as you know, adjectives in English are invariable. Look at these examples:

long plural form becomes → plural compound noun
[noun + noun]
100 trees with apples 100 apple trees
1,000 cables for telephones 1,000 telephone cables
20 boxes for tools 20 tool boxes
10 stops for buses 10 bus stops
4,000 wheels for cars 4,000 car wheels