Chromatic button accordions, familiarly known as CBAs, have buttons on both the left and right sides. Like piano accordions, they play the same note on the push and pull of the bellows. They have single treble notes on the left and typically have a Stradella bass (see piano accordions) on the right.
A CBA with at least five rows of treble buttons has the same scale pattern in any key. Many have buttons that are color-coded to correspond to the white and black keys of a piano.
Because some buttons are duplicates, a treble keyboard has more buttons than notes. The redundancy gives you a choice of where to play any of the duplicated notes.
There are several standard layouts for the treble side. Most common in Italy and on the streets of Paris is the C-system; in Russia and in Scandinavia, any one of several B-systems, which are more or less the mirror image of the C-system. One isn’t necessarily better or easier than the other; just different. They are versatile and powerful instruments, but learning aids are scarce, especially in English.
Castagnari Magica chromatic button accordion
Brandoni 181W chromatic button accordion
Layout of notes on a C-system instrument. A 4-row instrument has only one duplicate row.
Introduction to Concertinas and Accordions 