Hyphen In Compound Adjective With Numbers

When numbers are used as the first part of a compound adjective, use a hyphen to connect them to the noun that follows them. This way, the reader knows that both words function like a unit to modify another noun. This applies whether the number is written in words or in digits.

The president of the company gave a 10-minute speech to the Board of Directors.
He is knowledgeable in thirteenth-century politics.
The boy threw a rock at the second-story window.

However, a hyphen is not required if the number is the second word in the compound adjective.

He is a victim of Type 2 diabetes.
This elevator doesn’t go down to Basement 3.

Hyphen In Compound Adjective With Fractions

When using a fraction (e.g. half or quarter) as part of a compound adjective, it should be hyphenated so the reader understands which fraction is modifying which noun.

I half-wanted to commit a felony.
A quarter-million dollars is still a large amount of money.
You’ll need one-third of a pound of flour and one egg.
That’s a half-baked idea if I ever heard one!

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