Using Hyphens in Compound Adjectives

The Quick Answer
You can use a hyphen (or hyphens) to link the words in a compound adjective to show it is a single adjective. (A compound adjective is a single adjective that is made up of more than one word.) For example:
  • two-seater aircraft
  • never-to-be-forgotten experience

What Is a Compound Adjective?

A single adjective made up of two or more words is called a compound adjective. The words in a compound adjective can be linked together by a hyphen (or hyphens) to show they are part of the same adjective.

In the UK, your readers will expect you to use hyphens in compound adjectives.

In the US, your readers will be more lenient. The US guidance is as follows:
Use a hyphen if it eliminates ambiguity or helps your reader. If you're unsure, use a hyphen.

The Hyphen Might Be Essential

Sometimes, a hyphen is essential to avoid ambiguity. Look at these examples:
  • a heavy-metal detector
  • a heavy metal detector
Both versions above are correct, but they mean different things. The first device detects heavy metals. The second device detects metal, and the device is heavy. If we're talking about a device that detects heavy metals, then putting heavy metal detector would be wrong in the UK and the US.

Compound Adjectives with Numbers

The easiest compound adjectives to spot are the ones that include numbers. For example:
  • Two-seater aircraft
  • 4-bedroom house

"24-hour" (This is correct.)

"3-day" (This is correct.)
  • Three stone weakling
  • (Three-stone would be better.)
  • 15-page document

Compound Adjectives Without Numbers

Lots of compound adjectives do not include numbers. For example:
  • Philip is a far-too-chatty individual.
  • That was a never-to-be-forgotten experience.
  • James is a second rate plumber.

This should be should be "8-week money-back guarantee".

"Cambridge-based" and "high-speed" (both correct)

Often, a compound adjective consists of words that would not normally be joined together with a hyphen. For example:
  • The double glazing is still leaking. Can you call that double-glazing salesman?
  • (The words double glazing only need a hyphen when they are functioning as an adjective. In this example, the first time they are used, they are not an adjective. The second time they are used, they are an adjective describing salesman.)

  • You call this silver service? She's not a trained silver-service waitress.
  • (The second time they are used, the words silver service describe waitress. As they are a compound adjective, they are linked with a hyphen to show they are a single adjective.)

See Also

Alternatives to hyphens in compound adjectives Hyphens in prefixes Hyphens in compound nouns A list of common grammar errors A list of easily confused words A list of sayings and proverbs