Wrong: This is a high performance vehicle.
Right: This is a high-performance vehicle.
In numerous situations, a short phrase can be used in more than one way. When used as a compound adjective, it’s usually hyphenated; when used as an adverbial phrase it isn’t. For example, you might be traveling at high speed (adverbial phrase) in a high-speed (compound adjective) chase. Your new computer might have a dual-channel Ethernet controller that offers communications through dual channels. Or you might back up the car to go buy a backup generator.
Why do we care about hyphenating compound adjectives? For clarity. Many times it might not matter, but occasionally the lack of a hyphen can confuse the reader momentarily, forcing him or her to stop and reread the sentence. For example, does the phrase “four channel indicators” mean that there are four “channel indicators” or that the indicators are or a “four-channel" type? If the latter, using the hyphen will eliminate confusion. Whenever possible, a writer should try to avoid breaking the “reader’s trance.”
Compound adjectives don’t have to be limited to two words, either. Longer examples include “twenty-five-year-old man,” “better-than-average looking,” “faster-than-light drive,” and “ready-to-wear clothing.” (Note that in the latter two examples, without the hyphens we could be talking about something that’s faster than a “light drive”—whatever that is—and someone who is finally ready to “wear clothing.”)Want more? Check out my next blog. There are plenty of words still to come.