Early in the show, Cliff asks Bob about the band instrument he plays, which he calls a Tuba. Later, when Bob leaves home and comes to stay at the Romano/Cooper apartment, he walks in with a Sousaphone, which they continue to refer to as a 'Tuba'. While they are in the same category of instruments, a Tuba is a sitting-down instrument, while a Sousaphone is a walking (marching) instrument.