What is a Compound Adjective?

A compound adjective is created when two or more words work together to describe a single noun. To show the reader that these words act as one linked idea, we connect them with a hyphen.

If we don't use a hyphen, the sentence can become confusing because the reader might think the words are separate adjectives.

✅ Try our new sugar-free cookies! ❌ Try our new sugar free cookies.

By writing sugar-free, we make it clear that the cookies contain zero sugar, rather than being "free cookies made of sugar." Other common examples include ice-cold lemonade, open-plan living room, and world-famous pool.

Number + Noun Combinations

One of the most common types of compound adjectives combines a number and a noun. When this combination comes directly before another noun, two things must happen: you must use a hyphen, and the noun inside the adjective must be singular.

Even though the number is plural (like three, ten, or 500), the noun attached to it drops its "s."

✅ We took a three-hour boat ride down the river. ❌ We took a three-hours boat ride down the river.

✅ I have to write a ten-page essay on the history of cheese. ❌ I have to write a ten-pages essay on the history of cheese.

Note: If the measurement comes after the noun, it is no longer acting as a compound adjective. In that case, drop the hyphen and make the noun plural again: The boat ride was three hours long.

The "-ly" Adverb Exception

Not all compound adjectives get a hyphen. A major exception is when the first word is an adverb ending in -ly (like highly, beautifully, or poorly).

Because the -ly ending already clearly signals to the reader that the word is an adverb modifying the next word, a hyphen is unnecessary and grammatically incorrect.

✅ Guests will sleep in beautifully decorated suites. ❌ Guests will sleep in beautifully-decorated suites.

✅ Professor Higgins is a highly respected expert. ❌ Professor Higgins is a highly-respected expert.

Be careful: Adverbs that do not end in -ly (like well, ill, or high) still need a hyphen when they come before a noun. For example, a well-known author is correct, while a highly rated restaurant takes no hyphen.

Before vs. After the Noun

The placement of the adjective changes the punctuation. The general rule is that you only hyphenate a compound adjective when it comes before the noun it describes (attributive position).

If the exact same words come after the noun (predicative position), they usually lose the hyphen. This is a very common error for English learners.

✅ We are selling a well-maintained house. (Before the noun = hyphen) ✅ The house is well maintained. (After the noun = no hyphen) ❌ The house is well-maintained.

✅ She is a well-known blogger. ✅ The blogger is well known. ❌ The blogger is well-known.

When proofreading your own writing, always check where the noun is. If the descriptive words follow a verb like is, are, or seems, you can safely leave the hyphen out.