This article is part of our Complete English Grammar Practice Guide — explore all grammar topics with interactive exercises, including compound nouns.
English is full of words that team up to name a single thing. A toothbrush, a bus stop and a mother-in-law are all compound nouns — two or more words that have joined to express one idea. Compound nouns appear everywhere, from the kitchen (the washing machine, the frying pan) to the workplace (a job description, a notice period), so learning how they work pays off quickly.
This guide explains what compound nouns are, the three ways they are written, the main patterns used to build them, and the trickier points of stress, plurals and possessives. Throughout, the examples follow British English usage.
Key Takeaways
- A compound noun is two or more words acting as one noun: toothbrush, bus stop, mother-in-law.
- There are three written forms: open (bus stop), hyphenated (mother-in-law) and closed (toothbrush).
- The stress usually falls on the first part: a GREENhouse, not a green HOUSE.
- Plurals go on the main noun: toothbrushes, but passers-by and mothers-in-law.
- The possessive apostrophe goes at the end: my mother-in-law’s house.
What Is a Compound Noun?
A compound noun is a noun built from two or more words that together name a single person, place, thing or idea. The words combine into one unit of meaning, so a compound behaves like any other noun: it can be the subject of a sentence, take an article, and become plural.
In most compounds, the first word acts like a label that narrows down the second. The second word is the head — it tells you what kind of thing you are dealing with. A bus stop is a kind of stop; a swimming pool is a kind of pool; a toothbrush is a kind of brush. Reading a compound from right to left often reveals its meaning.
Ask “a kind of what?” The answer is usually the last word. A football is a kind of ball; a bedroom is a kind of room. That last word is the head of the compound.
The Three Written Forms
Compound nouns are written in three ways. The form does not change the meaning, but it does change the spelling, so it is worth learning frequent compounds as fixed items.
Open (spaced) compounds
Written as two separate words. These often involve longer or newer combinations.
bus stop, swimming pool, post office, washing machine, living room
Hyphenated compounds
The words are joined with a hyphen. Hyphens are common where words include a preposition or particle, or where joining them up would look odd.
mother-in-law, passer-by, runner-up, check-in, merry-go-round
Closed (solid) compounds
Written as a single word. These are usually short, familiar combinations that have fused over time.
toothbrush, bedroom, football, sunflower, haircut
There is no perfect rule for which form a compound takes, and many compounds shift over time. When unsure, check a trusted dictionary and stay consistent within a single piece of writing.
How Compound Nouns Are Formed
Although compounds combine many word classes, a few patterns cover the vast majority. Recognising these makes new compounds far easier to understand.
| Pattern | How it works | Examples |
|---|---|---|
| noun + noun | One noun describes another | football, toothpaste, bus stop, post office |
| adjective + noun | An adjective fixes onto a noun | blackboard, greenhouse, software, hardback |
| verb + noun | A verb combines with a noun | breakfast, pickpocket, washing machine |
| gerund + noun | An -ing word shows purpose | swimming pool, waiting room, frying pan |
| noun + preposition/particle | A noun joins a small word | passer-by, runner-up, check-in, mother-in-law |
Notice how the gerund + noun pattern names a purpose rather than an action in progress: a swimming pool is a pool for swimming, and a waiting room is a room for waiting. This is different from a continuous tense such as the children are swimming.
Stress: a GREENhouse vs a green HOUSE
One of the most useful clues to spotting a compound noun is stress. In the great majority of compound nouns, the stress falls on the first part. When the same two words are simply an adjective plus a noun, the stress moves to the second word.
a GREENhouse — a glass building for growing plants (compound noun)
a green HOUSE — a house painted green (adjective + noun)
a BLACKbird — a particular species (compound noun)
a black BIRD — any bird that is black (adjective + noun)
Stressing the first part of a compound makes your English sound natural and clear. Say a BUS stop and a SWIMMING pool, not a bus STOP.
Making Compound Nouns Plural
To make a compound noun plural, add the plural ending to the main noun (the head). In closed compounds and most open compounds, that head is the last word, so the change comes at the end.
toothbrush → toothbrushes
bus stop → bus stops
swimming pool → swimming pools
When the head comes first, that earlier word takes the plural — this is where learners often slip.
passer-by → passers-by
mother-in-law → mothers-in-law
runner-up → runners-up
passer-bys, mother-in-laws — common errors
A few compounds ending in -ful are flexible. Both spoonfuls and spoonsful are accepted, though spoonfuls is by far the more common form in modern British English. The same applies to cupfuls and handfuls.
Possessives of Compound Nouns
To show possession, add apostrophe + s to the end of the whole compound, never to the middle.
my mother-in-law’s house
the runner-up’s medal
my mother’s-in-law house — wrong; apostrophe in the middle
For a plural compound, make it plural first, then add the possessive to the final element: my brothers-in-law’s cars.
How Meaning Differs from the Separate Words
A compound noun usually has a single, fixed meaning that is more specific than the words on their own. The two words have fused into one concept, so you cannot always work out the meaning by adding the parts together.
As a compound (one concept)
- blackbird — a specific species of bird
- greenhouse — a glass building for plants
- darkroom — a room for developing photos
- heavyweight — a boxing weight class
As separate words (literal)
- black bird — any bird that happens to be black
- green house — a house painted green
- dark room — any room with little light
- heavy weight — an object that is heavy
Because the meaning is fixed, it is best to treat compound nouns as single vocabulary items — learning the whole compound, its spelling and its stress together.
Spelling Uncertainty and British vs American Differences
English compounds often drift from two words, to hyphenated, to a single word as they become more familiar. E-mail became email; both website and web site still appear. Because no single rule governs the change, dictionaries and style guides sometimes disagree.
| British English | American English |
|---|---|
| car park | parking lot |
| post box | mailbox |
| cash machine | ATM / cash machine |
| dustbin | trash can |
| mobile phone | cell phone |
Choose the spelling and vocabulary that match your reader’s variety of English, follow one trusted dictionary, and keep your choices consistent across a single document.
Common Mistakes to Avoid
two passer-bys → correct: two passers-by
my mother’s-in-law car → correct: my mother-in-law’s car
I bought a new tooth brush → correct: a new toothbrush (one word)
We swam in the swimming POOL → stress the first part: the SWIMMING pool
Practise Compound Nouns
Lock in the forms, plurals and meanings of compound nouns with our interactive exercises.
Start PractisingExercises to Practise on LexFizz
- Flash Cards — review common compound nouns with spaced repetition
- Quiz — multiple-choice questions on forms and plurals
- Match Up — match each first word to its partner
- Complete the Sentence — fill the gap with the right compound
- Word Search — find hidden compound nouns in the grid
Related Articles
Frequently Asked Questions
A compound noun is a noun made of two or more words that work together as a single unit of meaning. For example, toothbrush, bus stop and mother-in-law each refer to one thing, even though they are built from separate words. The first word usually describes or modifies the second, which carries the main meaning: a greenhouse is a kind of house, and a bus stop is a kind of stop. Compound nouns can be written as one word, two words, or with a hyphen.
There are three written forms. Open (or spaced) compounds are written as two separate words, such as bus stop and swimming pool. Hyphenated compounds join the words with a hyphen, such as mother-in-law and passer-by. Closed (or solid) compounds are written as a single word, such as toothbrush and bedroom. There is no perfect rule for which form a compound takes, so common ones are best learned individually and checked in a good dictionary.
Compound nouns are formed by combining different word classes. The most common pattern is noun plus noun, as in football and toothpaste. Other patterns include adjective plus noun (blackboard, greenhouse), verb plus noun (washing machine, breakfast), gerund plus noun where an -ing word describes purpose (swimming pool, waiting room), and noun plus particle or preposition (passer-by, runner-up). Once you recognise these patterns, new compounds are much easier to understand.
In most compound nouns, the stress falls on the first part. We say a GREENhouse (a building for plants) with stress on green, but a green HOUSE (a house painted green) with stress on house. This stress pattern is a reliable clue: if the stress is on the first word, the phrase is usually a compound noun with a single, fixed meaning. Saying compounds with the correct stress makes your English sound much more natural.
Usually you add the plural ending to the main noun. In closed and most open compounds this is the last word: toothbrushes, bus stops, swimming pools. When the main noun comes first, that word takes the plural: passers-by and mothers-in-law (not passer-bys or mother-in-laws). Some compounds with -ful are flexible, so both spoonfuls and spoonsful are accepted, though spoonfuls is far more common in modern British English.
To make a compound noun possessive, add apostrophe-s to the end of the whole unit, not to the first part. So we write my mother-in-law’s house and the runner-up’s medal, keeping the apostrophe at the end. For plural compounds, the rule still applies to the final element: my brothers-in-law’s cars. Placing the apostrophe in the middle, such as mother’s-in-law, is a common error to avoid.
A compound noun has a single, fixed meaning that is often more specific than the words alone. A blackbird is a particular species, not simply any bird that is black, and a greenhouse is a glass building for plants, not a green-coloured house. The words have fused into one concept. This is why we treat compounds as single vocabulary items and why stress and word order matter so much to their meaning.
English compounds often drift over time from two words, to hyphenated, to a single word as they become more familiar. Email was once e-mail, and today both website and web site appear. Because there is no single governing rule, dictionaries and style guides sometimes disagree. The safest approach is to follow a trusted dictionary, keep your spelling consistent within a piece of writing, and learn frequent compounds as fixed forms.
Yes. British and American English sometimes prefer different forms or words. British writers may keep a hyphen where American writers close the word up, and the two varieties use different compounds for the same idea, such as British car park and American parking lot, or British post box and American mailbox. When writing for a specific audience, choose the spelling and vocabulary that match their variety of English.
Common mistakes include pluralising the wrong part (writing passer-bys instead of passers-by), putting the apostrophe in the middle (mother’s-in-law instead of mother-in-law’s), and choosing the wrong written form (toothbrush is one word, not tooth brush). Learners also stress the wrong part, saying green HOUSE when they mean GREENhouse. Reading widely, checking a dictionary, and practising with exercises help you fix these errors.
Ready to master English grammar?
Explore All Grammar Exercises →