are three-note chord voicings that omit the fifths of the chords. The Belgian bass system is a variation used in Belgian chromatic accordions. It is also arranged in a circle of fifths but in reverse order. This system has three rows of basses, three rows of chord buttons allowing easier fingering for playing melodies, combined chords, better use of fingers one and five, and more space between the buttons. This system was rarely used outside of its native Belgium. Various free-bass systems for greater access to playing melodies and complex basslines on the left-hand manual and to forming one's own chords note-by-note. These are often chosen for playing jazz and classical music. Some models can convert between free-bass and Stradella bass; this is called converter bass. The free-bass left hand notes are arranged chromatically in three rows with one additional duplicate row of buttons. Luttbeg double-keyboard piano accordions have a piano keyboard layout on both the treble and bass sides. This allows pianists, most notably Duke Ellington, to double up on the accordion without difficulty. The Bercandeon is an improved version of that instrument, also making it a "keyboard bandoneon". Reed ranks and switches Inside the accordion are the reeds that generate the instrument tones. These are organized in different sounding banks, which can be further combined into registers producing differing timbres. All but the smaller accordions are equipped with switches that control which combination of reed banks operate, organized from high to low registers. Each register stop produces a separate sound timbre, many of which also differ in octaves or in how different octaves are combined. See the accordion reed ranks and switches article for further explanation and audio samples. All but the smaller accordions usually have treble switches. The larger and more expensive accordions often also have bass switches to give options for the reed bank on the bass side. Classification of chromatic and piano type accordions In describing or pricing an accordion, the first factor is size, expressed in number of keys on either side. For a piano type, this could for one example be 37/96, meaning 37 treble keys (three octaves plus one note) on the treble side and 96 bass keys. A second aspect of size is the width of the white keys, which means that even accordions with the same number of keys have keyboards of different lengths, ranging from for a child's accordion