Singular  Plural 
attorney-general attorneys-general
brother-in-law brothers-in-law
bystander bystanders
commander-in-chief commanders-in-chief
court martial courts martial
cupful cupfuls
father-in-law fathers-in-law
general staff general staff
go-between go-betweens
governor-general governors-general
handful handfuls
heir apparent heirs apparent
maidservant maidservants
man-of-war men-of-war
mix-up mix-ups
mother-in-law mothers-in-law
mouthful mouthfuls
notary public notaries public
passer-by passers-by
sergeant major sergeants major
sister-in-law sisters-in-law
son-in-law sons-in-law
step-parent step-parents
step-daughter step-daughters
step-mother step-mothers
tablespoonful tablespoonfuls
takeoff takeoffs