This is an adjective created when two or more adjectives modify the same noun.
- Blue-gray paint
- Absent-minded professor
- Semi-solid food
- Sweet-and-Sour pork
The idea is that the hyphen will help to avoid confusion. Otherwise, that tasty dish on the menu might confuse someone who’d never been to a Chinese restaurant before. Sour pork? And what kind of sweet comes with it? And what kind of food is “semi food,” any way?
You can also have compound adjectives created by an adverb modifying the adjective.
- Brightly-lit room
- Never-ending story
You do not need the hyphen if one (or more) of the words is capitalized.
- I’m raising my baby with the reliable Dr. Spock method
Although, if you wanted to use the tried-and-true Dr. Spock method, you’d need hyphens to hold the Tried and True words together.
Of course, over time, some words–held together so well by the hyphens–eventually merge into one, seamless word all their own. Like, say, “online.” Or “babysit.”
Then, of course, there are compound nouns, but . . . I’d bet you can figure those out on your own now, can’t you? Because, really, they’re pretty much the same thing, except that they’re nouns (grin). For some reason, “Snow-shovel” is springing to mind tonight, but that could be because of the weather forecast.
If you want to read more, look here for more information.