The left hand keyboard of most piano accordions and chromatic button accordions uses the standard bass system (also known as the stradella bass, manual 2 or simply MII). On a standard bass accordion, there are rows of buttons which offer the accordionist a selection of up to twelve bass notes and a selection of preset chords. This standard bass system is what is used to add the typical “Um paa paa” accompaniment often associated with so much music played on the accordion.
Some piano accordions and chromatic button accordions have a “free bass keyboard” on the left hand side of the instrument. It is so called because it frees the accordion and the accordionist from the restrictions of having a limited number of notes and fixed chords. Instead the free bass system offers a larger range (usually up to 58) of single notes. This type of keyboard is essential if you wish to play music from original piano and organ scores accurately making it a very serious consideration for anyone interested in playing classical music on the accordion although many folk and jazz musicians use this system also.
A full concert piano accordion or chromatic button accordion will have a standard bass keyboard and a free bass keyboard on the same accordion. The most common way of doing this is to have a switch that “converts” the chord rows on the accordion from preset chords into a free bass manual of single bass notes. Many accordion manufacturers also have a large range of smaller converter accordions with sizes suitable for children and adults. These accordions probably offer the most flexibility and allow the accordionist to play music in a wide range of styles with only one accordion although they can often be much more expensive than others.