A concertina is a free-reed musical instrument, like the various accordions and the harmonica. It consists of expanding and contracting bellows, with buttons usually on both ends, unlike accordion buttons, which are on the front.

A free reed aerophone is a musical instrument that produces sound as air flows past a vibrating reed in a frame. Air pressure is typically generated by breath or with a bellows. In the Hornbostel–Sachs system, it is number: 412.13. Free reed instruments are contrasted with non-free or enclosed reed instruments, where the timbre is fully or partially dependent on the shape of the instrument body, Hornbostel–Sachs number: 42.