You know, of course, that a phrasal adjective, or compound modifier — two or more words that combine to modify a noun — are usually hyphenated to signal that link (only before the noun, however, and not if, as with “income tax” and many other permanent compounds, the open compound is in the dictionary). Even though the relationship often seems obvious, this is language law. Phrasal adjectives, however, aren’t the only grammatical category in which hyphens are required even though they don’t seem necessary. Here are seven others:
1. Job Titles
Some job titles, such as secretary-treasurer, are hyphenated to signal the combined roles. Others, such as secretary-general (the title of the head of the United Nations), retain this form as a holdover from a time when hyphenation of compound nouns was rampant, though technically, general is an adjective modifying secretary (as in the example of president-elect, below). However, this usage is an anomaly: similar terms like “attorney general” and “major general” are open.
Note that the adjective+noun combination “vice president” is open, but some other such compounds are hyphenated (vice-consul) or closed (viceroy).
2. Compound Nouns
A handful of noun compounds stubbornly resist the usual usage evolution of open, hyphenated, and closed (or sometimes open to closed without the hyphenation middleman): The ones I can think of are by-product, life-form, light-year, and mind-set.
Many people treat these artificially preserved throwbacks incorrectly — the first and last compounds are often erroneously closed, and the hyphen is frequently omitted and a letter space inserted in the second and third ones — and why shouldn’t they? Omitting hyphens and treating these words as open or closed compounds doesn’t violate any scientific laws. However, until dictionaries respond to the attainment of a tipping point where most people are writing such terms incorrectly, these words should be hyphenated.
(Light-year may someday be closed, but because the first element of life-form ends with a vowel, it will likely remain hyphenated. On that note, the disinclination to close this type of open compounds affects other terms, such as shape-shifter.)
3. Compound Verbs
When you use two words together to refer to a single action, such as referring to air-conditioning a house, jump-starting a car, or mass-producing a product, a seemingly extraneous hyphen is required. (The same is true regardless of the form of the verb: air-condition and air-conditioned — but “air conditioning.”)
Hyphens in compound numbers such as twenty-one seem natural, but hyphenation of fractions (one-third) is counterintuitive. How many thirds? One. One is an adjective that modifies the noun third, so why hyphenate them unless they’re linking to modify a noun (“one-third full”)? I don’t make the rules; I just follow them.
Sometimes, prefixes you’d expect to be closed up to the root word are hyphenated, because closing them up would cause confusion with identical words with distinct meanings. Examples include resign/re-sign, resent/re-sent, and recreation/re-creation. (A rare case of a similar pair with a prefix other than re- is unionized/un-ionized.)
Generally, permanent compounds beginning with the prefixes all-, ex-, and self- are hyphenated: Examples include all-around, ex-governor, and self-control. (Selfish and selfless, as well as the unfortunate unselfconscious, are exceptions with self-.)
Some words beginning with co- (co-chair) and pro- (pro-choice) just look wrong closed up and are anomalously hyphenated, as are words in which the last letter of the prefix and the first letter of the root word are the same: anti-intellectual, co-owner, ultra-aggressive.
Words beginning with non- are almost always closed, but occasionally they’re seen hyphenated — and for good reason: “Nonlife-threatening injury” is an awkward treatment. Insert a hyphen when the prefix precedes a hyphenated phrasal adjective.
And why, if we refer to the early or late part of an era, such as a decade or a century (“early 1920s,” “late nineteenth century”), no hyphen is used, but a reference to the middle of a period requires one, as in mid-1970s or mid-century? Consistency would call for referring to “the middle 1970s” or “the middle of the century,” but mid- has replaced this usage.
Constructions such as president-elect may seem to be unnecessarily burdened with a hyphen, but they’re equivalent to modified phrases such as daughter-in-law. (Note, though, that such constructions do not include a hyphen when the first element is an open compound, such as in “vice president elect.”)
However, “editor in chief” shed its connective tissue long go without difficulty (though some people still incorrectly hyphenate the phrase), so similar constructions may follow suit. For now, though, go with the flow.