A chromatic button accordion is a type of button accordion where the right side keyboard has rows of buttons arranged chromatically. They have either three or five (or very rarely six) diagonally-horizontal rows. Each row can play three successive chromatic notes.
Chromatic accordions are most popular in Europe and in Russia. In Russia, this instrument is usually called a "bayan" (in Russian: баян).
There are two main types of button accordions. In both types, the further away from the top of the keyboard you go, the higher the notes; the closer to the top, the lower.
In the "type C" accordion, the middle "C" is on the row furthest to the right. In the "type B" accordion, the "C" is located on the third row from the right.
Most chromatic accordions have two extra rows located to the left of the main rows. They play the same notes as the same buttons of the 1st and 2nd rows. This can be helpful when playing a hard tune because you do not have to change fingerings when playing in a different key, but only move your hand to the left a row or two and play with the same fingering.
Some Serbian chromatic accordions have a sixth row, but this is not common.