Accordion/Left hand

The Stradella Bass System (also called the standard bass sometimes) is a type of button layout you find on the left (or bass) side of most accordions. It uses columns of buttons arranged in a circle of fifths.

Each vertical column has, in order from left to right:

The major third above (or minor sixth below, depending on which note) the root ("counter-bass")
The root note
The root note's major chord
The root note's minor chord
The root note's dominant seventh chord
The root note's diminished seventh chord

This is a chart of how a standard, 120-button bass layout will look like:

The middle "C" usually has a bump or hole in it for you to feel, to make it easier to find your way around the left buttons without looking at them.

Horizontal columns are arranged in a different order: every column goes up a fifth as you move up the keyboard, and down a fifth as you go down. So a "G" would be below middle "C", and a "F" would be above the middle "C".

Most full-size accordion basses have either 120, or, less commonly, 96 buttons. All notes in an octave are covered by both systems, so you can play in any key.

When playing with the bass buttons, you use all fingers but your thumb, which should stay on the board on the side of the accordion.