Many compound nouns present a challenge when it comes to determining how to convert them from singular to plural form. The solution usually seems simple enough — slap on an s — but the plural appendage doesn’t automatically go at the very end. Here’s the rule about how to figure out whether to write that, for example, during your last golf game, you shot two hole-in-ones or two holes-in-one. (After all, just because you’re lying doesn’t mean you should ignore proper grammar.)
The plural inflection — s or its variants — should be attached to the element of an open or hyphenated compound noun that changes in number. Hence, for example, “chief of staff” becomes “chiefs of staff,” not “chief of staffs” and “mother-in-law” becomes “mothers-in-law,” not “mother-in-laws.”
Some terms, however, are ambiguous: Should you write “attorney generals,” or “attorneys general”? The former treatment disregards that attorney is the key element; general, in this usage, is an adjective, not a noun. (This reverse placement of adjective and noun is a legacy of the French origin of the term.)
But “attorneys general” seems stilted and odd to many people, who prefer treating such compound designations like military ranks: A reference to more than one brigadier general, for example, would mention “brigadier generals,” not “brigadiers general,” even though general, recall, was originally a postnominative adjective.
However, similar terms are straightforward: “Secretary-elect” becomes “secretaries-elect,” and the plural form of “heir apparent” is “heirs apparent.”
Note that the rule does not refer to closed compounds, because in this type of compound, the element that changes number is invariably at the end: More than one headache involves multiple aches, not a plurality of heads (notwithstanding that reference can be made to more than one person having a headache at a time), mention of two or more copies of a handbook correctly emphasizes book, not hand, and households is likewise the correct form for describing more than one household.
But there are exceptions, as in reference to more than one passerby; that’s because, unlike the examples given above, this closed compound does not consist of two nouns combined in one word. Passersby, too, observes the general rule that the changeable element receives the plural inflection.
And what of compoundlike words formed from the combination of a noun and the suffix -ful? Dictionaries, responding to variable usage, list both a plural form in which the plural inflection follows the noun and a variant in which the s is appended after -ful. However, some people find the former structure awkward (handsful, teaspoonsful), while the -fuls form (handfuls, teaspoonfuls), to many, looks and sounds more logical.