Compound adjectives

Words working as one description

An adjective describes a noun, such as a person, a place or a thing. A compound adjective combines words into one description. A hyphen is the short mark - between parts, with no spaces beside it: low-cost.

• We found a low-cost way to repair the gate.
• There is a well-known café beside the theatre.

Low-cost describes something that does not cost much. Well-known means known by a lot of people. The words work together; they are not two separate descriptions joined by and.

Not every compound adjective contains a hyphen. Some are written as one word, such as homemade. Some descriptions use separate words, such as carefully packed. Cambridge notes that compound spelling varies and recommends checking a learner's dictionary. A hyphen is a spelling choice with guidelines, not the definition of a compound.

• We brought homemade biscuits for the journey.

Keep the difference from two independent adjectives: a small, quiet café is both small and quiet. Low-cost is one combined idea. See Adjective order and position for separate adjectives and their order.

A number and a unit before a noun

To describe a length of time, distance or another amount before a noun, put the number first and the unit second. In this pattern, keep the unit noun singular and join the parts with hyphens.

• We booked a four-night stay near the harbour.
• The house is at the end of a three-kilometre road.
• The shop sells two-litre bottles.

The singular unit belongs inside the compound: six-week, not six-weeks. The noun being described follows its own number: a six-week course, two six-week courses. Do not add an apostrophe to week in this compound.

• Both six-week courses start in September.

This rule covers the number-plus-unit adjective, not every noun before another noun. Some noun combinations contain a plural form. See Noun + noun: a bus driver, a car door, a shopping list rather than turning the singular-unit rule into a ban on all plural nouns before nouns.

Age: before the noun and after be

Before a noun, join the number, the singular time word and old: a four-year-old child. Month and day can take the place of year when you need a smaller unit.

• They restored a ninety-year-old clock.
• We adopted a four-month-old kitten.

After be, use the ordinary age phrase with separate words. For a number greater than one, the time noun is plural: four years old. You have changed the sentence pattern, not just removed the hyphens.

• Our neighbour's dog is thirteen years old.

The same contrast works with measurements: a two-metre-high wall, but the wall is two metres high. The plural in the full phrase does not belong inside the compound adjective.

High, low and other word combinations

Compound adjectives are not all made in the same way. Learn a compound with its meaning and a noun you can use it with. High and low can combine with a noun to describe a level or amount.

• The roof needs high-quality paint.
• She is looking for a part-time position at the museum.

Do not make the noun inside high-quality plural to match the following noun: high-quality windows. It describes their quality, not a number of qualities.

Some compounds end in an adjective: sugar-free means without sugar; open-minded describes someone willing to consider different ideas. A longer phrase can also work as one description, as in up-to-date information. Check the meaning as a unit rather than translating each part separately.

• We need up-to-date maps for the walk.
• The café serves sugar-free drinks.

Compounds with -ing and past participles

A participle is a verb form such as working, made or written that can be part of a description. A past participle can end in -ed, but forms such as made and known are irregular. Compound adjectives can combine another word with either kind.

• Sorting all these receipts is time-consuming.
• There was a handwritten message inside the box.
• We need a hard-working assistant for the summer.

Hard is not a noun in hard-working. The original words can have different jobs in different compounds. The useful questions are what the whole description means and how a dictionary writes it.

Do not swap an -ing form for a past participle simply because the words look similar. In handwritten, the item receives the writing; in time-consuming, the activity takes time. See -ed and -ing adjectives: feeling and effect for adjective meanings with -ed and -ing.

Hyphens before and after the noun

A hyphen can show which words belong together before the noun: a well-known name. Many such descriptions are written as separate words after be or another linking verb. A linking verb connects the subject to its description, as be does in the name is well known.

• We chose a well-known tune for the concert.
• That tune is well known here.
• Please send an up-to-date list of members.
• The list is up to date now.

This is not a rule to remove every hyphen after be. Compounds such as sugar-free and time-consuming can keep their hyphens in that position. The Canadian writing guide explicitly keeps hyphens in these noun-plus-adjective and noun-plus-participle patterns. Check the word, not just its place in the sentence.

• The new drink is sugar-free.
• This repair could be time-consuming.

Nor does moving a compound after be make a one-word spelling split apart: handwritten stays handwritten. Cambridge records both hard-working and hardworking, and both homemade and home-made. Use a consistent dictionary or house style; do not label a recorded spelling variant a grammar error.

When an -ly word describes a participle

An adverb can describe how something is done: carefully packed, recently painted. The Canadian and Australian writing guides leave the hyphen out when an adverb ending in -ly comes before a participle. This is a useful writing convention to follow.

• We unpacked a carefully wrapped mirror.
• There are newly planted trees beside the path.

This is a style convention, not proof that every hyphen you see in that position is ungrammatical. Chicago's advice also prefers no hyphen but acknowledges hyphenated practice elsewhere. Cambridge's compounds page itself includes environmentally-friendly. Follow the style you are using and check fixed words in a dictionary.

The ending alone is not enough: an adjective ending in -ly is not automatically an adverb. First ask what the word is doing. See Adverbs: forms and irregular adverbs for adjective and adverb forms.

Clear meaning and shared endings

Use hyphens to help your reader group the words correctly. The Australian Style Manual says dictionaries do not always agree; the aim is clear meaning within the style you follow. Do not add a hyphen to every pair of words before a noun. For example, the Canadian guide writes dark green paint without one.

• We chose dark green curtains for the bedroom.

You may see a shared ending written just once: three- and four-day trips means three-day trips and four-day trips. The hyphen after three shows that day is still to come. This is called a suspended hyphen. The Australian guide recommends repeating the words when that makes the sentence easier to follow.

• The centre offers two-day and five-day courses.

Both trip lengths or course lengths are separate choices. This is different from one compound with a number range. If the short version is hard to read, write the full descriptions instead.

What comes next

See Adjective order and position for separate adjectives, Noun + noun: a bus driver, a car door, a shopping list for nouns modifying nouns, -ed and -ing adjectives: feeling and effect for participle adjectives, and Comparatives and Superlatives for degree comparisons.