Some plural forms of compound nouns seem odd at first glance (e.g., attorney general becomes attorneys general rather than attorney generals). However, most of these compound nouns consist of the noun’s subject (attorney in the prior example) and its modifier (general). For these compound nouns, the plural should be built on the compound’s subject (usually the first word) rather than on the whole compound noun itself. The following list shows some examples formed in the above manner: 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.