Certain compound nouns form the plural on the noun’s subject rather than its modifier. Attorney general takes the plural attorneys general: the noun’s subject is attorney and its modifier is general. In such nouns the subject is often the first word, though not always (e.g., snow day becomes snow days). The following list shows some examples formed in the above method: attorneys general, brothers-in-law (the same holds for all relatives-in-law), courts-martial, holes in one, knights-errant, maids of honor, passersby, poets laureate, postmasters general.
Do not use the above method for noun + ful compounds, where the -s should be appended to the end of the entire word: bucketfuls, handfuls, spoonfuls, etc.