In English, words, particularly adjectives and nouns, are combined into compound structures in a variety of ways. These compound structures can change over time. A common pattern is that two words —for example, fire and fly — will be joined by a hyphen for a time — fire-fly — and then will be joined into one word — firefly. There is only one sure way to know how to spell compounds in English: use an authoritative dictionary.
There are three forms of compound words:
- the closed form, in which the words are melded together, such as firefly, secondhand, softball, childlike, redhead, keyboard, makeup, notebook
- the hyphenated form, such as daughter-in-law, master-at-arms, over-the-counter, six-pack, six-year-old, mass-produced
- and the open form, such as post office, real estate, middle class, full moon, half sister, attorney general.
Compounds are often hyphenated to avoid confusion. The New York Public Library’s Writer’s Guide points out that an old-furniture salesman clearly deals in old furniture, but an old furniture salesman would be an old man. We probably would not have the same ambiguity, however, about a used car dealer. When compounded modifiers precede a noun, they are often hyphenated: part-time teacher, fifty-yard-wide field, fire-resistant curtains, high-speed internet. When those same modifying words come after the noun, however, they are not hyphenated: a field fifty yards wide, curtains that are fire resistant, or The second-rate opera company gave a performance that was first rate.
Comparative and superlative forms of adjectives are hyphenated when compounded with other modifiers: the highest-priced car, the shorter-term loan. Adverbs, words ending in -ly, are not hyphenated when compounded with other modifiers: a highly rated bank, a partially refunded ticket, publicly held securities.
Sometimes hyphenated modifiers lose their hyphens when they become compound nouns: “A clear decision-making process was evident in their decision making.” This is not always so, however: a high-rise apartment building is also known as a high-rise.
When modifying a person with his or her age, the compounded phrase is hyphenated: my six-year-old son. However, when the age comes after the person, we don’t use a hyphen: My son is six years old. He is, however, a six-year-old.
Plurals and Possessives of Compound Words
Most dictionaries will give variant spellings of compound plurals. When you have more than one truck filled with sand, do you have several truckfuls or trucksful? Dictionaries will give you both, with the first spelling usually preferred. A dictionary will also help you discover that only one spelling is acceptable for some compounds — like passersby.
For hyphenated forms, the pluralizing -s is usually attached to the element that is actually being pluralized: daughters-in-law, half-moons, mayors-elect.
As a general rule, the plural form of an element in a hierarchical term belongs to the base element in the term, regardless of the base element’s placement:
- first sergeants
- sergeants major
- sergeants first class
- lieutenant generals
- lieutenant colonels
- deputy librarians
The possessive of a hyphenated compound is created by attaching an apostrophe -s to the end of the compound itself: my daughter-in-law’s car, a friend of mine’s car. To create the possessive of pluralized and compounded forms, it is best to avoid the apostrophe -s form and use an of phrase instead: the meeting of the daughters-in-law, the schedule of half-moons. Otherwise, the possessive form becomes weird: the daughters-in-law’s meeting, friends of mine’s cars.
This section does not speak to the matter of the possessive forms of compounded nouns such as “Professor Villa’s and Professor Darling’s classes have been filled.” See the section on possessives for additional help.
Compounds with Prefixes
With a handful of exceptions, compounds created by the addition of a prefix are not hyphenated. For example:
anteroom, antisocial, binomial, biochemistry, coordinate, counterclockwise, extraordinary, infrastructure, interrelated, intramural, macroeconomics, metaphysical, microeconomics, midtown, minibike, multicultural, neoromantic, nonviolent, overanxious, postwar, preconference, pseudointellectual, reunify, semiconductor, socioeconomic, subpar, supertanker, transatlantic, unnatural, underdeveloped
- Compounds in which the second element is capitalized or a number:
- anti-Semitic, pre-1998, post-Freudian
- Compounds which need hyphens to avoid confusion:
- un-ionized (to distinguish from unionized), co-op (to distinguish from coop)
- Compounds in which a vowel would be repeated (especially to avoid confusion)
- semi-independent, anti-intellectual (but reestablish, reedit)
- Compounds consisting of more than one word
- non-English-speaking, pre-Civil War
- Compounds that would be difficult to read without a hyphen
- pro-life, pro-choice, co-edited
Also, when we combine compound nouns, we use a hyphen with the first, but not the last: under- and overdeveloped nations.