Concertina

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.

There are also various duet system concertinas, which are much more rarely seen than Anglo and English concertinas. Their button layouts provide low notes in the left hand, high notes in the right, with some overlap (like a piano), and the same notes pushing and pulling.

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.