A square or rectangular free reed instrument, it is related to the Concertina. It is a direct descendant of the Chemnitzer concertina which where developed in the Saxony region of Germany independently of the English concertina.They are popular in Argentina in Tango orchestras, which the Tango was written for the Bandoneon originally. German concertinas (Koncertinas) where square rather than the British 'threepenny bit' in any case, though export versions where made in the six sided English style often. This gave rise to the Anglo-German concertina of modern music. Bandoneons where/are always square due their size and design. It is common to find a support point for a shoulder harness attached to a center frame point or rarely at one end.
Diatonic in nature, a different note is sounded on the pull and the push. There are keyboards on both sides of the instrument so that either hand can play melody. Chords are formed by playing adjacent notes. Most Bandoneons cover the base and treble clefts completely in range.
The instrument was designed by Herr Band and was originally intended to play mainly church music. It has since developed into an instrument used heavily in many forms of music (Folk and Classical) throughout Latin America, Spain, and Italy. The bandoneon is a relatively heavy instrument.
Pictured: A Bandoneon from Britten Sinfonia