A converter bass accordion with a converter switch in the left hand is probably the most versatile accordion type available. It is possible to obtain piano accordions and chromatic button accordions with a converter bass in the left hand.
As with most other accordions, a converter bass accordion has a regular standard bass (stradella) keyboard although on some Russian bayans the keyboard is modified slightly with the diminished chord row moved down one button.
There is usually a long converter switch in between the left hand buttons and the bellows and when this is pressed, the chord rows are “converted” into 3 or 4 rows of single bass notes. These bass notes usually have a range of up to 58 notes. Certainly in the UK, the most commonly found version is where the buttons are arranged chromatically over three rows in a similar fashion to the right hand keyboard on a chromatic button accordion.
The two most popular layouts for these are the B system (Russian bayan) system and the C system which usually go hand in hand with their right hand counterparts (take a look at our article on Chromatic button accordion systems to find out more). Occasionally converter bass accordions are made with only 3 rows of free bass notes but it is more usual to have a fourth row of buttons that repeats the notes in the first row which can make fingering choices a little easier.