In English, words, particularly adjectives and nouns, are combined into compound structures in a variety of ways. And once they are formed, they sometimes metamorphose over time. A common pattern is that two words fire fly, say will be joined by a hyphen for a time fire-fly and then be joined into one word firefly. In this respect, a language like German, in which words are happily and immediately linked one to the other, might seem to have an advantage. There is only one sure way to know how to spell compounds in English: use an authoritative dictionary.
How a word modified by an adjective "a little school," "the yellow butter" is different from a compound word " a high school," "the peanut butter" is a nice and philosophical question. It clearly has something to do with the degree to which the preceding word changes the essential character of the noun, the degree to which the modifier and the noun are inseparable. If you were diagramming a sentence with a compound word, you would probably keep the words together, on the same horizontal line.
Modifying 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 chase. When those same modifying words come after the noun, however, they are not hyphenated: a field fifty yards wide, curtains that are fire resistant, etc. 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. But this is not always the case: the most talented youngster. 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. The bluish grey was slowly disappearing from the bluish-grey sky. This is not always so, however: your 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.
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? The dictionary will give you both, with the first spelling usually preferred. (And the same is true of teaspoonfuls, cupfuls, etc.) The dictionary will 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. The Chicago Manual of Style says that "hyphenated and open compounds are regularly made plural by the addition of the plural inflection to the element that is subject to the change in number" and gives as examples "fathers-in-law," "sergeants-in-arms," "doctors of philosophy," "and courts-martial" (196). The NYPL Writer's Guide puts it this way: "the most significant word generally the noun takes the plural form. The significant word may be at the beginning, middle, or end of the term" (396). And then we get examples such as "attorneys at law," "bills of fare," chiefs of staff," notaries public," assistant attorneys general," "higher-ups," "also-rans," and "go-betweens."
Note: some dictionaries will list "attorney generals" along with "attorneys general" as acceptable plurals of that office. Whether that's a matter of caving in to popular usage or an inability to determine the "significant word" is unknown.
As a general rule, then, 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:
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, a writer is wise to avoid the apostrophe -s form and use an "of" phrase (the "post genitive") instead: the meeting of the daughters-in-law, the schedule of half-moons. Otherwise, the possessive form becomes downright weird: the daughters-in-law's meeting, friends of mine's cars.
One of the most difficult decisions to make about possessives and plurals of compound words occurs when you can't decide whether the first noun in a compound structure is acting as a noun that ought to be showing possession or as what is called an attributive noun, essentially an adjective. In other words, do we write that I am going to a writers conference or to a writers' conference? The Chicago Style Manual suggests that if singular nouns can act as attributive nouns city government, tax relief then plural nouns should be able to act as attributive nouns: consumers group, teachers union. This principle is not universally endorsed, however, and writers must remember to be consistent within a document.
This section does not speak to the matter of compounded nouns such as "Professor Villa's and Professor Darling's classes have been filled." See the section on Possessives for additional help.
With a handful of exceptions, compounds created by the addition of a prefix are not hyphenated:
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
Also, when we combine compound nouns, we would use a hyphen with the first, but not the last: when under- and overdeveloped nations get together. . . .
The following table presents a mini-dictionary of compound modifiers and nouns. Perhaps the best use of a very partial inventory like this is to suggest the kinds of words that a writer would be wise either to memorize or to be at least wary of. It is sometimes enough to know when we should get the dictionary off the shelf.
Notice that African American contains no hyphen, but Italian-American does. There are no hard and fast rules about this, and social conventions change. (There is no hyphen in French Canadian.) Some groups have insisted that they do not want to be known as "hyphenated Americans" and resist, therefore, the use of a hyphen, preferring that the word "American" be used as an adjective. Some resources even suggest that a term like Italian-American should be used only when the individual thus referred to has parents of two different nationalities. That's probably a stretch, but a writer must be aware that sensibilities can be aroused when using nationalities of any description. Consistency within a document is also important.
With a series of nearly identical compounds, we sometimes delay the final term of the final term until the last instance, allowing the hyphen to act as a kind of place holder, as in
Be careful not to overuse this feature of the hyphen; readers have to wait until that final instance to know what you're talking about, and that can be annoying.
The Chicago Manual of Style contains an extensive section devoted to compounded modifiers and nouns. That book's table of compounds categorizes compounds into various types, and helps us discover principles of spelling (and some really strange exceptions). Styles of compounding words change over the years, however, and writers might even find different versions in different dictionaries. The Chicago Manual is especially helpful because it tries to define the principles by which such decisions are made.
*The term "suspended compound" is borrowed from New York Public Library Writer's Guide to Style and Usage HarperCollins: New York. 1994. p. 416. Cited with permission.