B-system and the C-system (there are also regional variants). Rarely, some chromatic button accordions have a decorative right-hand keyboard in addition to the rows of buttons, an approach used by the virtuoso accordionist Pietro Frosini. Diatonic button accordions use a buttonboard designed around the notes of diatonic scales in a small number of keys. The keys are often arranged in one row for each key available. Chromatic scales may be available by combining notes from different rows. The adjective "diatonic" is also commonly used to describe bisonic or bisonoric accordions—that is, instruments whose right-hand-manual (and in some instances even bass) keys each sound two different notes depending on the direction of the bellows (for instance, producing major triad sequences while closing the bellows and dominant seventh or 7–9 while opening). Such is the case, for instance, with the Argentinian bandoneon, the Slovenian-Austro-German Steirische Harmonika, the Czech Heligonka Harmonika, the Italian organetto, the Swiss Schwyzerörgeli and the Anglo concertina. Piano accordions use a musical keyboard similar to a piano, at right angles to the cabinet, the tops of the keys inward toward the bellows. The rarely used bass accordion has only a right-hand keyboard, with ranks of 8', 16', and 32' reeds, with the lowest note being the deepest pitch on a pipe organ pedal keyboard (pedal C). It is intended for performing basslines in accordion orchestras. The rarely used piccolo accordion also has only a right-hand keyboard. 6-plus-6 accordions use a buttonboard with three rows of buttons in a "uniform" or "whole-tone" arrangement, generally known as a Jankó keyboard. The chromatic scale consists of two rows. The third row is a repetition of the first row, so there is the same fingering in all twelve scales. These accordions are produced only in special editions e.g. the logicordion produced by Harmona. Left-hand manual systems Different systems are also in use for the left-hand manual, which is normally used for playing the accompaniment. These usually use distinct bass buttons and often have buttons with concavities or studs to help the player navigate the layout despite not being able to see the buttons while playing. There are three general categories: The Stradella bass system, also called standard bass, is arranged in a circle of fifths and uses single buttons for bass notes and additional rows of single buttons for preset major, minor, dominant seventh, and diminished chords. The dominant seventh and diminished chords