A compound noun contains two or more words that join together to make a single noun. A compound noun is a type of compound word that is used to designate a person, place or thing. Compound nouns can be formed in several ways, which is one reason that the writing of these words in English can be tricky.
A compound noun is formed when two words are combined to make a completely new word. This means that the meaning of the new word must be significantly different than either of its parts. For example, a hot dog (or hotdog) - i.e., a type of sausage typically eaten on a bun - may be hot, but it's certainly not a dog. The combination of these two words means something entirely different than the mere combination of the adjective "hot" and the noun "dog," which would refer to an overheated, panting canine. Observe the difference below:
The first sentence uses "hot dog" as a compound word, but the second does not. The meaning is quite different in each sentence, and English speakers have no trouble telling the difference between pets and food!
Compound nouns, like all compound words, can be formed in three ways:
Most compound nouns contain at least one noun. The other word or words used for the compound noun may be an adjective, preposition or verb. The second word is often the "main" word, with the first word modifying it or adding to its meaning.
To get a sense of how compound nouns are formed, study the words below. We've listed the examples in groups that indicate the parts of speech that make up each compound noun.
There are no hard and fast rules concerning plurals of compound words, especially since some hyphens are omitted after time. In hyphenated words, usually the "s" goes at the end of the noun, as in daughters-in-law or mayors-elect. Sometimes it is at the end, as in go-betweens and higher-ups. In the open form, the "s" is often added to the noun, as in bills of fare, assistant secretaries of state, notaries public and full moons.
To make a compound word possessive, you usually add apostrophe + s to the end of the word, as in mother-in-law's car or five-year-old's birthday. If the compound word is plural, it can get a little strange with two "s" sounds close together, as in fathers-in-law's attire. If you can, it would be better to reword the sentence so the plural compound word does not need to be possessive: the attire of the fathers-in-law.
Compound nouns are just one way in which the English language has become richer over time. Once you master compound nouns, you may wish to learn about compound adjectives and compound verbs to round out your understanding of compound words.