Mother-in-Laws or Mothers-in-Law? How To Pluralize Compound Nouns

English is full of grammatical headaches. Plurals, for example, are a major hurdle to master for native speakers and language learners alike. Sometimes it’s easy: For many nouns, you can simply add an “s” to the end. (Think “dogs,” “teachers,” and “paintings.”) But other plural nouns, even those considered “regular,” have some changes in spelling: Nouns ending in “s” take “es” (as in “classes”), a noun ending in “y” often becomes “ies” (as in “parties”), and an “f” or “fe” ending often (but not always) turns into “ves” (“thieves” but “chiefs”). 

Irregular plurals tend to follow the direction of the language of origin, such as Latin (an alumnus turns into a group of alumni, and “appendix” is pluralized as “appendices”) or Greek (one criterion and many criteria). But even after you memorize the most common irregular plurals, questions arise: In particular, what do we do with compound nouns? 

A compound noun is made up of two or more words that together refer to a single person, place, or thing — think “toothbrush,” “editor-in-chief,” and “swimming pool.” Sometimes a compound noun is closed, meaning one word (as in “passerby”), and other times it’s hyphenated or open, with two or more separate words (such as a noun and an adjective or another qualifying noun). When you’re making a plural out of a compound noun, you usually apply the plural to the principal word, which is the main noun that’s being modified by the adjective — but it’s not always easy to determine which one that is. Here’s some help to figure out how to pluralize all types of compound nouns.

A closed compound noun looks like a single word, but it’s composed of two words  smushed together. The plural rules for closed compound nouns follow regular (and irregular) pluralization rules. 

Firefighter → firefighters

Notebook → notebooks

Stepchild → stepchildren

These examples follow the structure of pluralizing the principal noun in the word, with the appropriate plural ending coming at the end of the word. Of course, in the English language, there are always exceptions to the rule. An outlier closed compound noun is “passerby.” In this case, the person passing, the “passer,” is the principal noun, so the plural of “passerby” is “passersby,” with the “s” following “passer.” 

An open compound noun can also be called a noun phrase. It’s made up of two or more words and might include the preposition “of.” The principal noun placement is a little malleable: It depends on what’s being modified. 

Conflict of interest → conflicts of interest

Peace of mind → peace of mind

Attorney general → attorneys general 

Notary public → notaries public

High school → high schools

Data analysis → data analyses

An outlier in this case would be a term like “science fiction,” which is also a collective noun referring to the genre as a whole. It’s incorrect to say, “I like science fictions.” You need to specify what kind of science fiction you like: shows, novels, movies, or something else. The open compound noun becomes an adjectival phrase to modify another specific noun. “I like science fiction television shows” contains the adjectival phrase “science fiction” followed by the plural open compound noun “television shows.”

Hyphenated compound nouns are the trickiest compound nouns because there’s an inclination to apply the plural to the end of the noun, as you would for most closed compound nouns. But hyphenated nouns are a little closer in spirit to open compound nouns. If you’re married to someone with two mothers, they’re not your mother-in-laws, they’re your mothers-in-law. You still follow the rule of identifying the principal noun of the compound noun and applying the plural to the principal instead of to the adjective or modifier. 

Cul-de-sac → culs-de-sac

Runner-up → runners-up

Editor-in-chief → editors-in-chief

Artist-in-residence → artists-in-residence

As with every English rule, there are exceptions here. The hyphenated compound noun “check-in” should technically follow the rule of applying the plural to the principal, which is “check,” but instead we say “check-ins.” This might have developed because there can also be a verb form of “checks in.” 

For whatever compound noun you come across, pay attention to the principal noun for pluralization. Just know that there will always be some exceptions, and you’re not alone in your confusion.