This post discusses the three types of compounds in English: compound nouns, compound modifiers, and compound verbs.
Compound nouns come in three forms: closed, hyphenated, and open. They are formed by pairing multiple combinations of parts of speech, such as two nouns (workshop), a preposition and a noun (overlord), and a verb and an adverb (smackdown). Open compound nouns, which tend to be newer formations such as “life span” and “working class,” may consist of more than two words; these phrases are often adopted foreign terms such as “persona non grata” and “tour de force,” although phrases can also be compiled by combining two words into an open compound and then combining that set phrase with another, as when science and fiction team up and then unite with writer.
Hyphenation is usually a transitional phase between open and closed forms, but some words get stuck in this intermediary form; examples include by-product, light-year, life-form, and mind-set. Writers often style the these words as closed compounds, however, indicating that the closed forms may ultimately prevail. Hyphenated compound nouns that are likely to remain transitional include self-respect and well-being, although these, too, are sometimes erroneously treated as closed compounds.
Other hyphenated forms include compounds consisting of verbs connected to prepositions, resulting in nouns as go-between, follow-through, send-off, and start-up. (Startup is a common variation of the last word, mirroring words such as checkup and makeup, which until just a few decades ago were routinely written check-up and make-up, but start-up is still the favored form.) Hyphenated compounds, like open compounds, may consist of more than two words, as in the case of mise-en-scène and will-o’-the-wisp. Closed compounds include afterthought, caregiver, and lifetime. Forms of compound nouns are often arbitrary, and an element in common does not guarantee consistency, as shown in the examples “road trip” and roadblock.
Some compounds are formed from elements of words rather than full words, as in the case of the technological terms bit (from “binary digit”) and pixel (from “picture element”), which both pertain to units of data. Such words are sometimes formed in other languages from English vocabulary, as in the case of the Russian term kompromat (“compromising material”). However, common and proper nouns such as radar (formed from “radio detection and ranging”) and NASA (which stands for “National Aeronautics and Space Administration”) are considered acronyms, not compounds.
Similarly, compound modifiers, which describe a noun (and are often, as on this site, referred to as phrasal adjectives), may be open, hyphenated, or closed. Several categories of open compounds, which remain open rather than hyphenated even when they preced the noun they modify, exist. They include permanent compounds such as “post office” (as in “post office box”), which are identified as such by meriting their own dictionary entry in noun form; proper names such as “New York” (as in “New York subway system”); foreign terms adopted into English such as “de facto”; unambiguous phrases such as “Monday morning” (as in “Monday morning quarterback”); and constructions with least, less, more, and most (as in “the least important factor”). (But little, much, seldom, and often are connected to verbs with a hyphen to form compound modifiers.)
A rule of thumb for compound modifiers is to hyphenate if called for before a noun but leave open after a noun (for example “a dark-haired woman,” but “a woman who is dark haired”), unless, as in the case of such words as life-size, quick-witted, and stand-alone, the compound modifier is listed in a dictionary with a hyphen.
A combination of an adjective and a noun is often converted into a closed compound adjective. For example, “long time” (“a lengthy period”) becomes longtime (“lasting for a lengthy period”), and “every day” (“all days under discussion”) becomes everyday (“ordinary”). A related usage error that is increasingly pervasive is the lack of a distinction between “every day” and everyday; one often sees retail signage reading something like “Storewide savings everyday!” although the writer means “every day.” (“Everyday savings storewide” is correct, however, because here the term is employed as an adjective.)
Prepositions and adverbs, appearing in an open phrase such as “over all” (as in “The fence had fallen over all her flowers”) combine to form adjectives (as in “It fit an overall pattern”) or adverbs (as in “Overall, he was disappointed”).
Compound verbs are those formed from a verb and another part of speech to create a new verb. The five types of compound verb, listed with examples, follow:
Not all compound verbs are closed. Open compound nouns are sometimes pressed into service as compound verbs, becoming hyphenated in the process. Thus, for example, “spot check” (“a quick or random inspection”) becomes spot-check (“undertake a quick or random inspection”).