A compound noun contains two or more words that join together to make a single noun, like "keyboard." 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 why writing these words can be tricky. Let's go ahead and clear the air surrounding this topic.
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 individually.
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:
I want to eat a hot dog when we go to the county fair.
The hot dog was grateful to find a shady spot for a nap.
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 should have no trouble telling the difference between pets and food!
Compound nouns, like all compound words, can be formed in three ways:
Closed Form: These compound nouns simply push two words together to form a single word, with no additional punctuation or spaces. Examples include softball, redhead, makeup and keyboard.
Hyphenated Form: These compound nouns connect two or more words with one or more hyphens between them. Examples include six-pack, five-year-old, and son-in-law.
Open Form: These are compound nouns that function as one unique word, but are still written as two or more separate words, with a space between each component. Examples include post office, middle class, and attorney general.
Most compound nouns contain at least one noun. Of course, this isn't universally true, given words like takeoff, but it's a good point for consideration. The other word or words used to create compound nouns may be adjectives, prepositions, or verbs. The second word is often the "main" word, with the first word modifying it or adding to its meaning.
Truth be told, there are no rules governing why some compound words are hyphenated, some are closed, and others remain open. Sort of like our friend, the irregular verb, you simply have to study a word list or consult your dictionary.
In American English, if you're dealing with a common word, you can err on the side of closed form. If it's not a common word, then you might want to consider using a hyphen or open form. It's also worth noting that compound nouns that have become commonplace sometimes lose the space or hyphen over time. For example, "on-line" is now online. "Take-out" is now takeout.
While American English errs on the side of closed form, British/Canadian English errs on the side of a hyphen. For example, the Shorter Oxford Dictionary uses pot-belly while the American Heritage Dictionary uses potbelly.
Given the inconsistencies that revolve around compound nouns, it can be hard to know when to hyphenate. While the hyphenated form is less common than the closed form hyphenated compound nouns do have one benefit. They help avoid ambiguity.
For example, if you choose to write laughing-gas instead of "laughing gas," you're making it clear that the gas is not laughing and it's functioning as a compound noun.
Also, you may be able to avoid a proverbial slap on the wrist knowing that some words have more than one acceptable form. For example, we don't normally write "dry-cleaning" or "ink-well." Rather, we write dry cleaning and inkwell. But, you shouldn't get red-lined for using either form.
The best way to try to make some sense of this complex member of the English language is to get a sense of how compound nouns are formed. We've listed the examples in groups that indicate the parts of speech that make up each compound noun.
adjective/adjective: red-orange, turquoise blue, golden yellow
Red-orange is one of this year's most popular colors.
adjective/noun: blackboard, full moon, top hat
The teacher needed help erasing the blackboard.
adjective/verb: dry-cleaning, public speaking, wet sanding
Be sure to pick up the dry-cleaning on your way home.
noun/noun: toothpaste, wallpaper, fish tank
Toothpaste now comes in many flavors.
noun/preposition: love-in, hanger on, passer-by
During the 1960s, a love-in was a form of peaceful protest.
noun/verb: haircut, snowfall, photo shoot
Go get a haircut before the photo shoot.
preposition/noun: underworld, bystander, afterlife
Many mythologies feature an underworld.
preposition/verb: output, backbone, overthrow
The CEO wanted to increase her factory's output of new products.
verb/noun: swimming pool, breakwater, washing machine
I can't wait to dive into that swimming pool.
verb/preposition: takeout, check-in, drawback
Let's order takeout for dinner.
To no surprise, there are no hard and fast rules concerning the plural form of compound words, especially since some hyphens are omitted over time.
In hyphenated words, usually the "s" goes at the end of the noun, as in daughters-in-law or mayors-elect.
Sometimes the "s" 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. While your brain is wrapping itself around the subject, take a look at the cousins to compound nouns. Taking the time to familiarize yourself with compound adjectives and compound verbs will help you round out your understanding of compound words.