Hyphens are tricky punctuation marks. It’s hard to know when they are required and when they are merely optional. Part of the problem is that hyphenated words don’t always stay hyphenated. For example, until recently, the Associated Press required a hyphen in the word e-mail. It now prefers the simpler email. In 2007, the Shorter Oxford English Dictionary formally dropped hyphens from about 16,000 words, included bumblebee, leapfrog, and pigeonhole (formerly bumble-bee, leap-frog, and pigeon-hole).

One particular area of confusion is in compound adjectives. Generally speaking, compound adjectives that precede the word they modify are hyphenated, but be sure you know a genuine compound adjective when you see one.

A compound adjective is a single adjective composed of more than one word. Examples include: seven-year-old, six-pound, eight-foot, happy-go-lucky, well-intentioned, part-time, man-eating, Klingon-speaking, hard-hearted, left-footed, and ice-cold. When modifiers like this appear before the noun they modify, they are usually hyphenated. If they come after the modified noun, they are not.

• The four-story building is on fire.
• That burning building is four stories. 
• The right-handed batter hit a home run.
• The batter is right handed.

Be careful not to confuse compound adjectives with strings of discrete adjectives or with adverb-adjective combinations.

1. The dumb, old donkey fell asleep.
2. The bright red light was distracting.
3. The brazenly bold outlaw rode into town.
   

In example 1 above, dumb and old both modify donkey. They are distinct adjectives and don’t work together, therefore they need no hyphen. In 2, bright is an adverb modifying red. It’s not necessary to hyphenate in such a situation. The same goes, perhaps more obviously, in 3.

There are some exceptions to the adverb-adjective rule, however. Adverbs — even those ending in -ly — are sometimes hyphenated when combined with a past participle: the widely-known celebrity, the hardly-touched breakfast, the brightly-colored room. But this is an optional hyphenation. It is not required and I only recommend it when it serves to clarify meaning  and avoid awkwardness.

Adverbs like well and fast, which can also work as adjectives, are most often hyphenated before the noun: well-lit room, fast-acting medicine, well-known author, fast-drying paint. Without the hyphen, it’s possible to confuse the reader into thinking the words are discrete adjectives (Is the paint fast and drying or is it fast-drying?).

And this leads to the most important general principle regarding hyphenated modifiers: Use hyphens to avoid confusion. If there is no risk of confusion, it is often acceptable to omit them.

The Grammar Monkeys from the Wichita Eagle demonstrate the need for confusion-avoiding hyphens with a dose of humor:

We need hyphens because…

• a small-business owner is not the same as a small business owner.
• a heavy-equipment operator is not the same as a heavy equipment operator.
• forty-year-old women are the not same as forty year-old women.
• an anti-child-abuse program is not the same as an anti-child abuse program.

At the same time, it is usually acceptable to omit hyphens when there is little to no chance of the reader misconstruing your meaning. Grammar Girl points this out with a sentence like “I’m using my noise cancelling headphones.” Noise cancelling is clearly a compound adjective, but since there is little chance of misunderstanding, few but the most rigid prescriptivists would insist on a hyphen. Personally, I prefer one here for the sake of consistency, but doubt I’d notice its absence if someone else had written it.

In summary, when it comes to adjectives, hyphen use isn’t guided by hard-and-fast rules. Keep these general principles in mind and use your instincts.

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