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. Due to this, the free bass system is used more often to play serious classical music with less transcription involved.
Most bass systems can switch between the Stradella and free-bass systems, while some accordions have free-bass only or auxiliary free-bass rows in addition to the Stradella buttons (sometimes called a bassetti).
There are two common free-bass systems: one is the chromatic free bass, where the notes move in a chromatic pattern upwards or downwards (see chart at right). The other is the quint bass system, which has a note system similar to the Stradella bass layout, but spread over several octaves. The former system gives you around five octaves of range in the left hand, and the latter about three. Usually, only big and expensive accordions have free-bass and it is still a relatively new concept; chances are the instrument you have, or are looking to buy, does not have it. As a beginning accordionist, you should not be too worried about it.
The study of free bass is currently beyond the scope of this module, although it may be covered at a later date. Knowing the free-bass system is not essential to becoming a professional accordionist; many pieces can be played just as well on the Stradella system, although free bass is useful for complex pieces like piano music or three/four voice counterpoint (which can frequently be played note-for-note without any transcription even necessary).