The left hand of an accordion is usually used to accompany the melody played on the right hand.
The buttons on the left hand are divided into two categories:
Unlike the right hand, where each key is linked to a pallet (or 2 in the case of concert accordions), pallets on the left hand are not tied to a particular key.
A mechanical system allows a number of pallets to be operated when a key is pressed.
For example, the key corresponding to C major will operate three pallets, that of C, E and G. The key corresponding to G major will operate G, B, and D pallets.
This system provides rich accompaniment capabilities with very easy access for the musician, since one key can play a full chord.
In addition, accordion left hands have multiple voices, allowing each note to be played over multiple octaves. For comparison, a piano requires the use of both hands to play a multi-octave chord.
On the accordion this is done by simply pressing a key, and the accordionist does not have to think about the notes that make up the chord.
This complex left-hand mechanism allows the musician to easily play accompaniment patterns alongside the melody played on the right hand.