ThinkTank #6 - English
A compound adjective is an adjective that contains two or more words.
In general we put a hyphen between two or more words (before a noun) when we want them to act as a single idea that describes something. This adjective with two words joined by the hyphen is called a compound adjective.
When we have compound adjectives using a number as the first expression, the word referring to the number is in SINGULAR form and is joined to the number with a hyphen.