It could play a 46 note chromatic scale. The Chromatic Button Accordion is not diatonic, and probably has the greatest range of treble notes of any accordion available today. Their size ranges from an accordion with 20 treble keys and 12 bass buttons, to the modern Chromatic Button Accordions which have up to 6 rows of treble buttons and 160 bass buttons.
The chromatic keyboard has more than one system. It can have anywhere between three and six rows of small round (and sometimes rectangular) keys. The fourth, fifth and sixth rows are repeats of the first three rows, to increase fingering possibilities. The two most common layouts are the B and C systems. The B system is better for technically challenging works, whereas the C system is easier to play chords and play melodic music. The six row chromatic is used almost exclusively by Yugoslav players and is also known as the Serbian accordion.
The bass system can be stradella, free bass or convertible between the two, the free bass facility giving an overall range of up to 11 octaves. This makes it possible to perform serious classical or contemporary works. The chromatic accordion is probably most widely used in Russia.There it is called the Bayan.