The functions the hyphen appear to be straightforward, but exceptions and inconsistencies abound. This post serves as a guide to the recommendations of The Chicago Manual of Style regarding hyphenation.
Hyphens are often introduced when new noun compounds are created, including in technological vocabulary, but such terms usually become closed compounds, though there are exceptions, such as mind-set and light-year. Other exceptions include constructions with certain first or second elements, such as in the case of self-respect and president-elect, and noun combinations such as city-state and writer-director.
Some terms that include letters linked to nouns retain hyphenation (A-list, T-bone, X-axis). Omission of a hyphen in email is trending, but similar terms such as e-commerce resist this evolution. Some prefixes take hyphens (anti-inflammatory, “non-Hodgkin’s lymphoma,” and nouns preceded by ex, such as ex-partner), though most prefixes do not require them. Hyphens also represent expression, in isolation, of a prefix or a word element (pre-, -er) when construction of a word using that prefix would otherwise not require a hyphen. Another use of hyphens is with words to be distinguished from nonhyphenated homographs (co-op, re-creation).
Other special cases for hyphenation with nouns include relationship terms preceded by great (great-grandmother) and in-law as well as combinations using in-law (sister-in-law), some compound nouns beginning with vice (vice-consul), constructions ending in odd (hundred-odd), and terms for compound nationalities where the first element is altered to end in o (Anglo-American), but not others that are no so altered (Italian American).
Hyphens link some double and even triple and quadruple surnames (“Lobelia Sackville-Baggins”), though not all double-, triple-, or quadruple-barreled surnames, as they are also called, are hyphenated (“Sacha Baron Cohen”). They also sometimes connect double first names, as in “Jean-Paul,” although this usage is rare in English names. Some company and product names use hyphens (Bristol-Meyers Squibb, EZ-Kleen).
Hyphens are employed in spelled-out numerical terms representing different place values (twenty-four), in fractions (as in one-third), and in number sequences, such as phone numbers and Social Security numbers, and number groupings, such as dates styled entirely in numerals.
One of the most common uses of hyphens is in words in phrasal adjectives preceding but not following a noun (“short-term investment,” “off-the-cuff remark”) and when combining similar-looking constructions that begin with comparative adverbs such as better, much, and well (“best-kept secret”).
Some style manuals (but not this site) recommend that phrasal adjectives be hyphenated regardless of their position, and a few such expressions (such as far-reaching) are always hyphenated regardless of position or style authority. Also, a letter space should never intervene when a hyphen connects two words or numbers, except when suspending the first use of a word common to two or more phrasal adjectives (“fifteen- and thirty-day increments”).
If a hyphenation links one word with an open compound, use an en dash rather than a comma to clarify that the symbol links the word to the entire compound, not just the element of the compound adjacent to the symbol: “pre–ice age migration,” “post–World War I recovery,” “mountain lodge–style ambience,” “Stephen Curry–level ball handling.” (The rule does not apply to abbreviations standing for open compounds, so use a hyphen, for example, in “US-Mexico border.”) If a compound is already hyphenated, use an additional hyphen to connect a word or prefix, as in “non-English-speaking actors” (extending from “English-speaking actors”).
When are hyphens used erroneously? When adverbs ending in -ly are mistakenly attached to the words that follow (as in “richly-detailed design”) and when adjectives are wrongly hyphenated to nouns (“near-term”). And although verb phrases are often hyphenated (test-drive), those words, as used in “I’m taking it for a test drive,” do not constitute a verb phrase.
When in doubt about whether to insert or omit a hyphen, consult a dictionary or a style manual, or check a publication’s or organization’s house style guide if you are writing for one.
Hyphens are also employed to break a word across two lines of type. Such breaks should occur between syllables, as demonstrated in a dictionary, but many publications choose to avoid this use of hyphens for aesthetic and practical reasons. If they are employed, it is recommended that type be adjusted so that no more than two end-of-line hyphens appear in a row and that they not be used at the end of the last line of a column or a page. (In addition, words that already include a hyphen should not be broken across two lines of type except at the existing hyphen.) Also, they are not advised for headlines and other large-type elements.
An issue related to hyphenation is capitalization of hyphenated terms in headlines and titles. Capitalize the following elements according to the recommendations of The Chicago Manual of Style:
• the first element
• subsequent elements except for articles; prepositions; coordinating conjunctions (and, but, for, nor, and or); and flat, sharp, and the like following letters denoting musical keys and chords
• the second element if it is a proper noun or proper adjective following a prefix or similar form that does not stand by itself as a word (Anti-, Pre-, and so on)
• the second element in a hyphenated spelled-out number (Fifty-One, Twenty-Fifth, and so on) or hyphenated simple fraction (“Two-Thirds Vote”).