The hyphen, along with its cousins the en and em dash, may be the most misunderstood punctuation mark in English. Hyphens are used to join parts of a word or compound phrase, as in ex-wife, full-length mirror, and by-the-book negotiations. As the Chicago Manual of Style puts it, “Far and away the most common spelling questions for writers and editors concern compound terms—whether to spell as two words, hyphenate, or close up as a single word.”
Hyphens: here today, gone tomorrow
One reason that hyphenation is so complex: it changes over time. A tour through the Google nGram view of many common words reveals their hyphenated predecessors: co-operate became cooperate, to morrow became to-morrow and then tomorrow, and good-bye became goodbye (though both are still acceptable). Some modern terms, like website/web site, have recently settled their usage wars, but email/e-mail is still anyone’s guess. Email is gaining popularity, as is eBook, as compared to e-book and ebook. A dictionary definition is often the best place to go to seek clarification, or at least understand your options.
Another reason hyphenation can be confounding is that the best practices of usage leave room for interpretation. The main function of hyphens in compound modifiers, or groups of words working together to modify a noun, is to eliminate ambiguity of meaning. For example, in the phrase “up-to-date technology,” we hyphenate “up-to-date” to signal that these three words are to be read as one concept, or adjective, functioning to modify the word “technology.” This way, nobody will make the mistake of reading this phrase as an expression of readiness to go on a romantic date with technology, or as a readiness to mark technology with the day, month and year. In another example, a “heavy-metal detector” detects heavy metals (or perhaps heavy-metal music), but a “heavy metal detector” is a metal detector that is heavy. However, ambiguity can be a subjective matter; as the Chicago Manual states: “Where no ambiguity could result … hyphenation is not needed.”
Some rules about ages and adverbs
Age terms are another stumbling point for many. The hyphenation in the following two examples is appropriate: I have to babysit my three-year-old cousin. He has a five-year-old. However, in the following sentence in which the age comes after (rather than before) the noun it modifies, no hyphenation is needed: Sheila is seven years old. One trick of the trade is to look for the plural of “years” in such constructions. If the word “years” is plural, chances are the construction does not need hyphenation.
Compound modifiers in which the adverb ends in -ly do not take a hyphen, as in overly thorough exam. In addition to numerous sections devoted to hyphens, the Chicago Manual also offers a 10-page hyphenation table, which any good copy editor finds indispensable.
Love learning about punctuation? What’s the difference between the colon and the semicolon?