A concertina is a small accordion-like instrument. Instead of buttons on one side and a piano-like keyboard on the other, it has buttons on both sides.
There are two common kinds:
- The Anglo concertina (from "Anglo-German") has buttons in curved rows following the fingertips. Pushing and pulling the bellows give two different notes from the same button. It is the ancestor of the bandoneon.
- The English concertina has buttons in a rectangular arrangement of four staggered rows, with the short side of the rectangle at the wrist. Pushing and pulling give the same note. A scale in most keys alternates between one side and the other.
Needless to say, a player of one of these "systems", given a concertina of a different system, will feel like s/he is playing an entirely new instrument.