Grammar Basics: Compound Nouns

The usage of compound nouns often appear confusing to students. Let’s understand with the help of an example.
Million dollar smile or million dollars smile?
How many of us are guilty for the confusion in this - is it million dollar smile or million dollars smile? not many, because this is something that has been taught since childhood, however, what we do not know is the reason as to why it is million dollar smile and not million dollars smile, another example of similar words is - 5 star hotel, and 5 star chocolate.
What are Compound Nouns?
Compound nouns are words for people, animals, places, things, or ideas, made up of two or more words. Most compound nouns are made with nouns that have been modified by adjectives or other nouns.
In many compound nouns, the first word describes or modifies the second word, giving an insight into what kind of thing an item is, or providing us with clues about the item’s purpose. The second word usually identifies the item.
Look at the following sentence:
The contractor decided to construct a 40 house/houses building.
This statement might cause a little bit of confusion. The compound noun here should be: forty house building.
Let’s analyse some of the compound nouns:
Million dollar smile - million dollar is the adjective, while smile is the noun.
5 star hotel - 5 star is the adjective, while hotel is the noun.
40 house building - 40 house is adjective, while house is the noun.
Thus, the basic rule is adjectives can never be plural and hence the adjective part of the compound noun is always singular.
A compound noun generally forms its plural by adding ‘s’ to the principal word. Given below is a list of compound nouns along with their plural form:
List of compound nouns:
Singular Plural
commander-in-chief commanders-in-chief
coat-of-mail coats-of-mail
son-in-law son-in-law
step-daughter step-daughters
maid-servant maid-servants
man-servant men-servants
passer-by passers-by
looker-on lookers-on
man-of-war men-of-war
spoonful and handful spoonfuls and handfuls