A compound noun is a noun built from two or more words that work together as a single unit of meaning. Bedroom, ice cream, and mother-in-law are all one thing each — even though one is written solid, one with a space, and one with hyphens.
The parts of a compound don't have to be nouns themselves. What matters is that the whole combination names one specific person, place, object, or idea, and behaves as one word in a sentence.
Three ways to write them
The same compound can appear in three different written forms. English has no single rule that decides which form is correct for a given word — you check a dictionary.
| FORM | HOW IT LOOKS |
|---|---|
| Closed | one solid word |
| Open | two separate words |
| Hyphenated | joined by hyphens |
Examples for each form:
- Closed: bedroom, toothbrush, notebook, football, sunflower
- Open: ice cream, high school, living room, post office
- Hyphenated: mother-in-law, check-in, merry-go-round, twenty-one
The three forms are not random categories. They are stages on a timeline. A new compound usually starts as two words, then picks up a hyphen as it becomes familiar, and finally fuses into one word once it's fully established. E-mail became email; web site became website. This is why dictionaries disagree — they're catching the language at different points in the same slow process.
One sentence, three compounds, three different spellings. All of them are correct.
How they are formed
Compounds are built from almost any combination of word classes. Six patterns cover most of what you'll meet.
| PATTERN | EXAMPLE |
|---|---|
| noun + noun | bedroom, bus stop |
| adjective + noun | blackboard, fast food |
| verb + noun | swimming pool, washing machine |
| noun + verb | sunrise, haircut |
| verb + preposition | check-in, make-up |
| preposition + noun | underworld, onlooker |
Noun + noun is by far the most productive pattern. English speakers invent new ones freely whenever a concept needs a short label — screen time, data breach, podcast host — and most of these start life as open compounds.
Note that the first element usually stays singular even when the meaning is plural. A tree with many apples is an apple tree, not an apples tree. A box for tools is a tool box, not a tools box.
Making them plural
Most compound nouns form the plural the normal way — add -s or -es to the end of the whole word.
Hyphenated compounds with a clear main noun are different. The plural ending goes onto the principal word, not the end of the compound. The principal word is the one that carries the core meaning: a mother-in-law is primarily a mother, so that's the word that takes -s.
| SINGULAR | PLURAL |
|---|---|
| mother-in-law | mothers-in-law |
| brother-in-law | brothers-in-law |
| passer-by | passers-by |
| runner-up | runners-up |
| commander-in-chief | commanders-in-chief |
A few compounds have no obvious principal word and take a plain -s at the end: grown-ups, go-betweens, forget-me-nots. A few others stay the same in singular and plural: aircraft, spacecraft, headquarters.
Stress pattern
Stress is what separates a compound noun from an ordinary adjective-and-noun phrase in speech. A compound noun carries the main stress on the first element; a regular phrase stresses both words about equally.
In the examples below, the capitalized syllable is the one that carries the stress.
The same test works with other pairs:
This pattern is consistent enough to rely on. If a native speaker stresses both words equally, they mean two separate ideas. If they compress the stress onto the first word, they mean one thing with one name.
A small group of compounds breaks the rule, usually in titles and place names: Prime MINister, Mount EVerest, New YORK. These follow the stress pattern of regular phrases even though they name one specific thing.
Common mistakes
Plural on the principal word
Whole-word plural
First noun stays singular
Missing hyphens
Wrong form: closed vs open
Wrong form: hyphenated vs open