If there is one thing that sets accordions apart from other free reed instruments it is the ability to play full chords with the press of a single button. This idea is developed to its fullest potential with the accordion’s standard bass system. This standard bass accordion system also known as stradella bass, manual II or just MII, is thought to have developed towards the end of the nineteenth century
The left hand keyboard or “bass keyboard” of most piano accordions, chromatic button accordions and bayans use this standard bass system sometimes with some slight regional variations (notably with French and Belgian accordions and Russian bayans). Some larger diatonic accordions also use the standard bass system.
The standard bass is probably the main thing that makes an accordion able to play the music that we think of as “accordion music”. Whether that’s a Scottish waltz, an Irish jig, a French musette tune or even a classical piece written for this system, the standard bass accordion is almost certainly an essential feature.
The two inside rows nearest to the bellows of the accordion are bass notes (the “um” of Um pa pa); these are usually low in pitch. Using these buttons the accordionist can select any one of twelve different notes (one for each note of the chromatic scale). There are many repeated notes making it possible to play the same note in different places along the keyboard.
With the remaining four rows, the accordionist can play the preset chords. Moving out from the bellows of the accordion, each row of buttons produces major, minor, dominant seventh and diminished seventh chords respectively. When the accordionist presses one button, a mechanism of rods and pegs inside the instrument selects all three notes of the chosen chord.
A full size standard bass accordion usually has 120 buttons arranged over six rows. A 72 bass accordion has the same amount of notes and chords as a full size 120 bass accordion. Although some accordions with more buttons don’t have the diminished chord row, most do and so more buttons simply make it easier to play in a wider range of different keys. Accordions with less buttons can have less chord rows which can limit the variety of chords available. Other accordions with less buttons have all the chord types but in less keys limiting the number of keys that an accordionist can play in.