Most dictionaries will give variant spellings of Plurals of Compounds. 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:

first sergeants



sergeants major



sergeants first class



colonel generals [Russian]



lieutenant generals



lieutenant colonels



apprentice, journeyman, and master mechanics



deputy librarians



deputy assistant secretaries of state

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.