A Comprehensive Guide to Forming Compounds

A compound is a word or word group that consists of two or more parts that work together as a unit to express a specific concept. Examples are double-check, cost-effective, around-the-clock, hand-to-hand, forward-thinking, eyeliner, and iced tea. They might also be formed from prefixes or suffixes, as in ex-president, supermicro, presorted, shirtless, or unforgivable.

Basically, compounds are written in one of three ways: solid (teapot), hyphenated (player-manager), or open (which ranges from phrases such as off and on or little by little to combinations like washing machine—have a field day finding more). Because of the variety in formation, the choice among the styles for a given compound represents one of the most vexing of all style issues writers—and lexicographers—encounter.

For some terms, it is often acceptable to choose freely among open, hyphenated, and solid alternatives, even though the term has been used in English for an extended period (for instance, lifestyle, life–style, or life style). Although the styling that ultimately takes hold for a compound may be determined by nothing more than editorial and writerly preference, there are patterns of new compounds as they become established in the English language. Compound nouns, for instance, are usually written as one word; compound verbs are generally written as two; compound adjectives are often written with a hyphen. But note that we added "usually," "generally," and "often"—we're hedging. (Be advised that we'll be using noncommittal terms throughout, and, essentially, that's the point of the following articles: there aren't fast rules to forming compounds, but there are patterns.)

Compound adjectives are combinations of words that work together to modify a noun—technically, they work as unit modifiers. As unit modifiers, they are distinguished from other strings of adjectives that may also precede a noun. For instance, in the constructions "a low, level tract of land" or "that long, lonesome highway," the two adjectives each modify the noun separately. We are talking about a tract of land that is both low and level and about a highway that is both long and lonesome. These are regarded as coordinate modifiers.

In the examples "a low monthly fee" and "a wrinkled red shirt," the first adjective modifies the noun plus the second adjective. In other words, we mean that the monthly fee is low and the red shirt is wrinkled. These are noncoordinate modifiers. In the example "low-level radiation," we do not mean radiation that is low and level or level radiation that is low; we mean radiation that is at a low level. Both words are working as a unit to modify the noun—thus, they are unit modifiers.

Unit modifiers are mostly hyphenated. Hyphens not only make it easier for readers to grasp the relationship of the words but also aid in avoiding confusion. For example, the hyphen in "a call for a more-specialized curriculum" removes any ambiguity as to which the word more modifies, and the hyphen in re-sign distinguishes it from resign. Other examples are co-ed and coed, shell-like and shelllike, over-react and overreact, co-worker and coworker, which have either consecutive vowels, doubled consonants, or simply an odd combination of letters and which the inclusion of a hyphen aids in their readability.

Preposition/adverb (particles) + noun compounds are styled solid, especially when they are short and the first syllable is accented followed by a syllable with falling stress (as in afterthought, crossbones, download, offhand, upstairs, outfield, onstage, overseas, underhand). There are also hyphenated particle compounds, like in-house, off-the-cuff, off-line (or offline), and on-line (or online).

The styling of Internet (internet?)–related compounds (e-mail/email, website/web site) remains in flux, with the same compound styled different ways in different publications. We continue to be eagle-eyed lexicographers in our Western Massachusetts-based aerie.

Compounds—new, permanent, and temporary—are formed by adding word elements to existing words or by combining word elements. In English, there are three basic word elements: the prefix (such as anti-, non-, pre-, post-, re-, super-), the suffix (as -er, -ism, -ist, -less, -ful, -ness), and the combining form (mini-, macro-, pseudo-, -graphy, -logy). Prefixes and suffixes are usually attached to existing words; combining forms are usually combined to form new words (photomicrograph).

For the most part, compounds formed from a prefix and a word are usually written solid (superhero). However, if the prefix ends with a vowel and the word it is attached to begins with a vowel, the compound is usually hyphenated (de-escalate, co-organizer, pre-engineered). But there are exceptions: reelection, cooperate, for example. In addition, usage calls for hyphenation between a prefix and a capitalized word or number (post-Colonial, pre-19th century).

A prefixed compound that would be identical with another word, if written solid, is usually hyphenated to prevent misreading (re-creation, co-op, multi-ply). Prefixed compounds that might otherwise be solid are often hyphenated in order to clarify their formation, meaning, or pronunciation (non-news, de-iced, tri-city). Also, such compounds formed from combining forms like Anglo-, Judeo-, or Sino- are hyphenated when the second element is an independent word and solid when it is a combining form (Judeo-Christian, Sino-Japanese, Anglophile).

Some prefixes, and initial combining forms, have related independent adjectives or adverbs that may be used where the prefix might be expected. A temporary compound with quasi(-) or pseudo(-), therefore, might be written open as modifier + noun or hyphenated as combining form + noun. Thus, the writer must decide which style to follow (quasi intellectual or quasi-intellectual; pseudo liberal or pseudo-liberal).

Compounds formed by adding a suffix to a word are usually written solid (yellowish, characterless), except those having a base word that has a suffix beginning with the same letter or is a proper name (jewel-like, American-ness). Then, there are unique formations such as president-elect and heir apparent. Additionally, when a word is used as a modifier of a proper name, it is usually attached by a hyphen ("a Los Angeles-based company," "a Pulitzer Prize-winning author").

Most two-word permanent and temporary compounds (unit modifiers) are hyphenated when placed before a noun ("one-way street," "a risk-free investment," "East-West trade agreements," "blue-gray/bluish-gray paint") but are often open when following a noun ("The author is well known").

Permanent compounds are those that are so commonly used that they have become—need we say—permanent parts of the language. Temporary compounds are created to meet a writer's need at a particular moment, and they are often formed of an adverb (such as well, more, less, still) followed by a participle, and hyphenated when placed before a noun ("a still-growing company," "a more-specialized operating system," "a now-vulnerable opponent"). Temporary compounds, often formed from an adverb ending in the suffix -ly followed by a participle, may sometimes be hyphenated but may also be open because adverb + adjective + noun is a normal word order ("an internationally-known artist," "a beautifully illustrated book").

Temporary adjectival compounds may also be formed by using a compound noun. If the compound noun is an open compound, it is usually hyphenated so that the relationship of the words to form an adjective is immediately apparent to the reader ("a tax-law case," "a minor-league pitcher," "problem-solving abilities"). If readily recognizable, the units may occur without a hyphen ("a high school diploma" or "a high-school diploma"; "an income tax refund" or "an income-tax refund"). Also, if the words that make up a compound adjective follow the noun they modify, they fall in normal word order and are, therefore, no longer considered unit modifiers that require hyphenation ("The decisions were made on the spur of the moment"; "They were ill prepared for the journey"; "The comments were made off the record"; "I prefer the paint that is blue gray").

When a noun + noun compound is short, and established in the English language and pronounced with equal stress on both nouns, the styling is likely to be open (bean sprouts, fuel cell, fire drill). Many short noun + noun compounds, however, that begin as temporary open ones and have the first word accented tend to become solid (database, football, paycheck, hairbrush); this is also the case for some adjectives (shortcut, drywall—but then there's red tape and red-hot). There are also compounds formed from a verb followed by a noun that is its object, and they tend to be styled as solid (carryall, pickpocket). Vice versa, there are noun compounds consisting of a verb form preceded by a noun that is its object (fish fry, eye-opener, roadblock), and adjective + noun compounds that are written open (genetic code, minor league).

Writers also use a hyphen to make the "unit" relationships of nouns immediately apparent (English-speakers, Spanish-speaking students, fund-raiser, gene-splicing), but compounds in which a noun is the object of a following verb-derived word tend to be written open (problem solver, air conditioning).

Finally, when the nouns in a noun + noun compound describe a double title or function, the compound is hyphenated (city-state, secretary-treasurer, hunter-gatherer, bar-restaurant). And compounds formed from a noun or adjective followed by man, woman, person, or people, as well as denoting an occupation, are regularly solid (congresswoman, salespeople). We're pretty sure about those guidelines.

These compounds may be hyphenated or solid. The compounds with two-letter particles (such as by, to, in, up, on) are most frequently hyphenated since the hyphen aids in quick comprehension (lean-to, trade-in, add-on, start-up). Compounds with three-letter particles (off, out, through) are hyphenated or solid with about equal frequency (spin-off, payoff, time-out, follow-through, giveaway).

And then there are the verb + -er + particle compounds and verb + -ing + particle compounds. Except for established words like passerby, these compounds are hyphenated (hanger-on, runner-up, listener-in, falling-out, goings-on, talking-to). There are also the two-word established forms consisting of a verb followed by an adverb or a preposition, which is styled open: set to, strike out. Then we have words composed of a particle followed by a verb that are usually styled solid (upgrade, bypass).

The verb form of a compound noun (whether open or hyphenated) most often is spelled with a hyphen (field-test, water-ski, rubber-stamp), whereas a verb derived from a solid noun is written solid (mastermind, brainstorm, sideline). That one's simple enough. Phew.

That is the question, especially when it comes down to adverb and adjective compounds. And the stickler's answer is to hyphenate when the modifier is before the word it modifies and to write the compound in open form when it follows it (since there is little or no risk of ambiguity). For example, a journalist might publish a word-for-word quotation or a person might be quoted word for word by the journalist, or a writer might be told that what is said is off the record, and any off-the-record information is to remain confidential. However, usage evidence shows that this formula is not closely followed: a team could play back-to-back games or play two games back-to-back; a boss and employee might have a face-to-face discussion or talk face-to-face; a candidate's position might be middle-of-the-road; a child could be accident-prone like his or her accident-prone parent. The point is: many permanent and temporary compounds keep their hyphens after the noun in a sentence if they continue to function as unit modifiers.

But compound adjectives composed of foreign words are not hyphenated when placed before a noun unless they are always hyphenated ("per diem expenses," "the a cappella chorus," but "a ci-devant professor"). Also, chemical names used as modifiers before a noun are not hyphenated ("a citric acid solution"). And a compound noun having three or more words may be either hyphenated or open, depending on preference and usage evidence: editor in chief, base on balls, give-and-take, good-for-nothing, know-it-all, justice of the peace, jack-of-all-trades, pick-me-up, stick-to-itiveness.

Hyphens are sometimes used to produce inflected forms of verbs that are made of individually pronounced letters or to add an -er ending to an abbreviation—although apostrophes are more commonly used for the purpose (x-ed vs. x'd, you decide).

From the time the American League first allowed designated hitters in 1973, another 41 years passed before the first DH was inducted into the Baseball Hall of Fame. Frank Thomas will finally get some company this weekend when Edgar Martinez and Harold Baines join him in Cooperstown, New York. Three Hall of Famers in 46 years is a powerful testament to the challenge of DH-ing.
— J. P. Hoornstra, The Orange County Register, 17 July 2019

His continued growth as a player will be key to NU's secondary growing into one of the league's best, and Jackson has the right kind of coach, former NFL-er Travis Fisher, to push him toward it.
— Sam McKewon, The Omaha (Nebraska) World-Herald, 2 Aug. 2019

A hyphen separates prefixes composed of single letters, numerals, or letter-numeral combinations from the rest of a chemical term. In addition, italicized prefixes are followed by a hyphen. The hyphen is also used to separate units of certain chemically complex terms:

α-amino-β-(p-hydroxyphenyl)propionic acid

2-methyl-3-ethylpentane

6H-1,2,5-thiadiazine

In amino acid sequences, hyphens are used to separate the abbreviations ("Ala-Lys-Pro-Thr-Tyr-Phe-Gly-Arg-Glu-Gly").

It should be noted, however, that most chemical names used as modifiers are not hyphenated ("the amino acid sequence," "sodium hypochlorite bleach").

Numbers that form the first part of a compound modifier that express measurement are followed by a hyphen ("a 28-mile trip," "a 10-pound weight," "a nine-pound baby"), or that are used in a ratio ("a fifty-fifty chance," "60-40 chance"). An adjective that is composed of a number followed by a noun in the possessive is not hyphenated ("two weeks' notice," "a four blocks' walk"). Also, when the modifier follows a noun, it is usually not hyphenated ("The teacher required an essay that was five pages"; "Children who are twelve years old and under can order from the menu"; "The fence is 12 feet high").

Hyphens are used in fractions (e.g., two-thirds), and they join the parts of whole numbers (twenty-one). The hyphen is also found in serial numbers, and social security or engine numbers. If you're measuring something, you might also use the hyphen (foot-pound, kilowatt-hour, column-inch, light-year), or if you are talking about periods of time ("pre-2000" or "post-2000," or "post-20th/twentieth century").