Arguing about whether to hyphenate two or more words that jointly modify another is surely one of the most tedious things about being a copyeditor. Making matters worse, many people have a vague sense that hyphenation in general is on the way out, but not so many people understand when this idea is relevant and when it isn’t. As a public service, then, I take keyboard in hand to outline which of these hyphens are required, which are incorrect, which are open for discussion, and why that discussion should not involve fisticuffs.
In a few types of compound modifiers, hyphens are simply wrong. Those in which the first element is an -ly adverb, for example, are never hyphenated in U.S. English: a beautifully decorated cake. Multi-word proper nouns or foreign phrases are not hyphenated when used as modifiers: New York lawyer, de facto segregation. Neither are chemical compounds: carbon dioxide emissions. Sums of money written in combinations of symbols and words do not use hyphens (a $150 million loss), even though fully spelled-out amounts do (a two-million-dollar house). In all these constructions, the elements of the compound are firmly stuck together by something other than a hyphen—capital letters, foreign language, and so on.
In many more cases, however, hyphens are virtually always used, by convention even if not by strict logical necessity. A compound ending with a present or past participle is hyphenated before the noun it modifies: ear-splitting scream, moth-eaten sweater, twice-baked potatoes. Sometimes this is necessary for clarity: man eating shark or government financed projects might look like a subject-verb-object sequence. But modifiers that are formed by adding -d or -ed to a noun are invariably hyphenated as well, even though blue-eyed boy is no more possible to misconstrue than beautifully decorated cake. It’s like using your right hand for a handshake—this is just the way it is. Compounds joining a number to a noun or adjective are also hyphenated: three-mile hike, five-pointed star, 40-year-old virgin. So are fractions that modify nouns: two-thirds majority. Open compound verbs must be hyphenated when they serve as modifiers: walk-on part. Compounds involving comparatives and superlatives such as longer and longest are hyphenated (longer-term loan), but usually not those with more, most, less, and least (a more perfect union, though perhaps more-loyal citizens to distinguish greater degree from greater quantity).
This list of “never” and “always” categories isn’t exhaustive; the guidelines in The Chicago Manual of Style, Words into Type, and Edward D. Johnson’s Handbook of Good English come closer to comprehensiveness. But the point is that these are decisions you don’t have to agonize about. When it comes to compound modifiers, some hyphens actually are right or wrong.
Those that aren’t are generally the hyphens joining (or not) compound modifiers that fall into one vast category: noun phrases used adjectivally before another noun. Civil rights movement, orange juice carton, income tax laws, and high school students are familiar examples. A few publications would hyphenate every one of these compounds, because their styles require all those hyphens that are not forbidden—a tidy, ultra-consistent approach that makes life pretty easy. At a few other publications, all would be left open, on the grounds that permanent noun compounds never need hyphens even when they aren’t capitalized or foreign. (I’ve heard this called the “peanut butter rule.”) This, too, is simple and consistent. But most editors have to make case-by-case decisions based on how familiar a term is and how readily a word that’s modifying the other part of the compound could be misconstrued as modifying the main noun instead.
Some styles hyphenate most of these compounds but make exceptions for the most familiar: since we all know what civil rights are, the likelihood is remote that someone would picture a “rights movement” whose tactics are always polite. Others leave them open as a default but hyphenate compounds that genuinely force a reader to guess: is banana bread pudding made with banana bread, or does it just have banana slices in it? In all cases, though, the familiarity of a compound depends on the subject and the audience. Hyphenating terms that are common in a particular field may look naïve or odd in writing aimed at people in that field. But that doesn’t mean it’s “wrong,” especially in writing aimed at a more general audience. Leaving these terms open when they could confuse your readers isn’t going to make your writing seem any more authoritative.
What troubles many copyeditors about such approaches is that they could lead to inconsistency. I suspect, though, that most readers don’t think about consistency the same way editors do—and not all editors think about it the same way, either. I once encountered a style in which all but the most ambiguous noun + noun modifiers were left open (peanut butter sandwich) but even the most familiar adjective + noun modifiers were hyphenated (high-school student)—a pattern that’s consistent on its own terms but looks thoroughly inconsistent to anyone with the more usual “peanut butter rule” in mind. Leaving compounds open if they’re in the dictionary is consistent—to someone who’s using the same dictionary as you, and who understands that this is why watercolor-painting classes is hyphenated but oil painting classes is not. Short of the hyphenate-everything approach, in fact, just about any system you devise risks looking inconsistent to someone. Such rules may be useful for our own convenience, but tearing our hair out over whether they truly yield consistency is usually wasted energy that could be better spent arguing about the serial comma.