Compound Nouns Explained: How to Use Compound Nouns

A compound noun is a grammatical construction that combines two separate words to create a new word or phrase that functions as a noun. For example, the words “washing” and “machine” combine to form the compound noun “washing machine.” Compound words can consist of an adjective and a noun, a verb and a noun, or a preposition and a noun. Sometimes, multiple words combine to form a single word, while in other cases, the terms remain separate.

The second word of a compound noun will often identify the main noun, while the first word modifies it. In the compound noun “sunglasses,” “sun” modifies “glasses,” creating a new noun that refers to a specific kind of glasses. Broadly, there are three different ways to form compound nouns in English grammar:

1. 1. Open form: The first is called open form, and it involves two words that appear next to each other in a sentence. Although separate terms, they will combine grammatically to form a single idea. “Swimming pool” and “hot dog” are examples of open compound nouns.
2. 2. Closed form: The second category is closed-form compound nouns, wherein the words in the compound noun will combine to form a new word, such as “toothpaste” and “campfire.”
3. 3. Hyphenated: The third kind of compound noun is a hyphenated compound noun, and, as the name implies, a hyphen connects the words. Examples include “dry-cleaning,” “hanger-on,” and “son-in-law.”

A compound noun must function in the sentence as a noun—meaning it must be a person, place, or thing—and follow the grammatical rules for nouns. Compound nouns can serve as the subject or object of a sentence. Consider these four ways to use compound nouns:

• Adjective + noun: A compound noun can involve both an adjective and a noun, as in “goldfinch.” The adjective “gold” modifies the noun “finch.”
• Verb + noun: A verb may combine with a noun to create a new compound noun, such as “rainfall” or “snowmelt.”
• Preposition + noun: Various other constructions are possible using prepositions and nouns, such as in “overstory,” “undertones,” or “bystander.” Note that some of these types of compound nouns can be made in the reverse order, as in “passerby.”
  

When pluralizing nouns, the standard procedure is to add an “s” to the end of the word. “Sock” becomes “socks,” “tree” becomes “trees,” and so forth. With compound nouns, the rules are slightly different:

• Closed form: When you pluralize compound nouns in the closed form, such as “sparkplug,” adding an “s” to the end is usually correct: “sparkplugs.”
• Open form: When the noun is an open form, the “s” placement depends on the parts of speech involved in the compound noun. Usually, the noun itself gets the plural modification, while the modifying word remains singular. For example, “full moon” becomes “full moons” and “bill of sale,” becomes “bills of sale.”
• Hyphenated compound noun: For hyphenated nouns, add an “s” to the principal word.For example, “mother-in-law” becomes “mothers-in-law.”
• Exceptions: The pluralization rules for compound nouns are not definitive; practice and familiarity can be the best guide. For instance, the plural form of “hanger-on” is “hangers-on,” but the plural of “check-in” is “check-ins.”

As in many areas of English grammar, the rules for using compound nouns are not always clear-cut. Consider these two common mistakes when forming compound nouns:

1. 1. Open vs. closed compounds: A common mistake is making a compound noun closed when it should be open (for example, writing “coldcuts” instead of “cold cuts,” or writing “nevermind” instead of “never mind”). The opposite can also happen, such as writing “suit case” in place of “suitcase,” or “home work” instead of “homework.”
   
2. 2. Pluralization: Pay attention to the placement of the “s” in compound nouns. For example, the plural of the compound noun “inspector general” should be “inspectors general,” not “inspector generals.”

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