This is probably a better and more detailed explanation:
The standard 120 bass accordion is composed of forty single bass notes and eighty chords. They are arranged in a series of six rows, each containing twenty buttons. The second row contains Fundamental Bass notes. All bass and chord buttons are gauged from this row, as each bass is a key-note to the relative chord buttons below. The first or top row are counter bass notes, each being two full tones, or a major third higher than the Fundamental bass directly below. This arrangement places all the notes of the scale within easy reach of the fingers, as both rows are used for bass runs. The third row is composed of major chords; the fourth row of minor chords; fifth row of dominant seventh chords; sixth row of diminished seventh chords; each in the same relative position to the fundamental bass.
Each short row of buttons placed at an angle, contains bass notes and chords related to one key. Beginning from C, each consecutive row down is in the key of one added flat.
Example: C, F, Bb, Eb, Ab, Db, Gb Cb E, A
Up from C, each consecutive row is in the key of one added sharp.