The free-bass is a type of keyboard system that is found on some accordions. Unlike the Stradella-bass system, all of the buttons on a free-bass keyboard play single notes, not chords. The free-bass also has a bigger range of notes than the Stradella system, and it is easier to play out a tune with it. Because of this, the Free Bass system is used more often to play serious classical music, especially counterpoint (when two or more melodies are being played at the same time).
Some bass systems can switch between the Stradella and free-bass systems.