What Is a Compound Noun?

Compound nouns are grammatical nouns, which is a person, a place, or a thing, that contain at least two words joined together for the best definition. They can include only nouns or a noun paired with another part of speech, such as an adverb, a preposition, or an adjective. Referred to as either open, closed, or hyphenated, the type of compound noun plays an important role in determining the plural form of the compound.

At its most basic form, a noun represents a person, place, or thing. The words boy, church, and cup are all examples of nouns. A compound noun must contain at least one noun, but it can be paired with any other part of speech. If paired with another part of speech, the noun doesn't have to appear as the first word in the compound.

A compound noun can be formed with two nouns, or a noun with another part of speech. The words alone each hold their own meaning, but, when joined together into a compound noun, it creates another meaning. For example, the two nouns tooth and paste can be compounded to form the word toothpaste. Another example is the word sunrise , which contains the noun sun and the verb rise. Finally, moving the noun to the second position, the word underground has the preposition under in front of the noun ground.

There are three different types of compound nouns: closed, open, and hyphenated. The three examples given in the above paragraph are all closed compound nouns because there is no space between the words. An example of an open compound noun is deputy sheriff because the two words remain separate. The final category are hyphenated compound nouns, such as sister-in-law, which also demonstrates a compound noun that uses more than two words.

Knowing the type of a compound noun is important in determining pluralization, which often includes identifying which word is the most significant — the base word or head word, meaning the word that doesn't describe or modify. The last word in a closed compound becomes plural, so sunrise becomes sunrises. In open and hyphenated compound nouns, the most significant word is pluralized. For example, in the open compound noun deputy sheriff, the word sheriff is the most significant word since deputy is describing the rank of sheriff — it becomes deputy sheriffs. However, in the hyphenated compound noun sister-in-law, the word sister is the most significant and the other two modify it, so the plural version is sisters-in-law.

There are, however, special exceptions to these pluralization rules for compound nouns. Word meaning can make a difference. For example, sales clerks is used instead of sale clerks, for example, because sales means selling while sale means a price discount.

Nouns that only exist as plural nouns will stay plural in a compound. For example, clothes hanger become clothes hangers, since the first term was already plural. When it isn't clear how a compound should be pluralized, a dictionary should be consulted.