Some words and phrases seem to confuse people when it comes to making them plural. Here are some of the most often confused.
mother-in-law: mothers-in-law NOT mother-in-laws (this goes for any in-law)
car of tomorrow- cars of tomorrow NOT car of tomorrows – NASCAR announcers drive me crazy with this one
passerby: passersby NOT passerbyers
doctors’ office: when referring to an office with more than one doctor. First you make doctor plural (doctors) then show possession by adding ” ‘ ” (doctors’). This applies to any noun. boys’ club, dogs’ dish but you must be referring to more than one boy, dog, doctor etc.