The Stradella bass system is the standard left-hand layout on 120-bass accordions. It arranges 20 columns of root notes along the circle of fifths, with 6 rows per column: bass note, counter-bass (a major third above), major chord, minor chord, dominant 7th chord, and diminished chord. ChordColor renders this entire 120-button grid with interval coloring, so you can see the harmonic content of every button at a glance -- something that is nearly impossible on the physical instrument, where you navigate by touch memory alone.
The circle-of-fifths layout means that adjacent columns are always a fifth apart. This is why common chord progressions (I-IV-V, ii-V-I) fall under the fingers so naturally on accordion -- they are literally next-door neighbors on the button board. The interval colors make this relationship visible: you can see how the root of one column matches the fifth of the column next to it.
The two bass rows (bass and counter-bass) provide single notes for bass lines, while the four chord rows produce full chords from a single button press. ChordColor shows what notes each button actually triggers -- the major chord button plays a three-note major triad, the 7th button plays a four-note dominant seventh. Understanding these voicings helps accordion players anticipate how their left-hand accompaniment interacts with the right-hand melody.