Plurals of Compound Words (Solid, Hyphenated, and Multi-Word)

Compound words can be solid, hyphenated, or multiple words — there is no way to determine which form is correct except by looking up the word in the dictionary. For solid compound nouns, add the plural to the end of the word (roommate→roommates, bookkeeper→bookkeepers). Exception: the only solid word where the plural is inserted in the middle is passerby→passersby.

For hyphenated or multi-word compound nouns that contain a main noun, make the main noun plural (editor in chief→editors in chief, brother-in-law→brothers-in-law, cross-section→cross-sections).

For hyphenated or multi-word compound nouns with no main noun, add the plural to the end (go-between→go-betweens, look-alike→look-alikes, know-it-all→know-it-alls).