The keyboard mechanism is concealed under the removable top, which is exposed in the German design (Deutsche Bauform).

The three-row B-fingering was used around 1870. Its extension by two auxiliary rows to the five-row keyboard was added from around 1890. The melody side of this instrument is unisonoric, the bass side bisonoric.

A pressed key with a raised valve, which releases the tone hole to which the chamber adjoins.

The C/B fingering keyboard has several advantages over a piano keyboard. It allows a relaxed hand and arm position when playing melodies and the distances to be bridged when playing chord fingerings are shorter due to the narrower key arrangement. Wide finger spreading in an unfavourable hand position is thus avoided.

The mechanism on the left side are also located inside the housing.

The lonely button operates the air flape, the other 12 form the bass manual. This is divided into a single note and a chord section (upper 6 keys). In contrast to the melody side, the bass side is bisonoric.

The mechanism, the levers (clavis) and valves are located under the removed cover. The air flap on top.

Two soundposts (upright chamber) are mounted on the back. The lower higher part with the larger 16' reeds belongs to the single tones of the keynotes, the upper smaller reeds to the chord area.

The tone holes can be seen where the soundpost has been removed. On the other non-visible side, the valves rest on them.

The single reedplates typical of an accordion with the reeds attached to them are clearly recognisable. On the back of the soundposts there are another three large and five small single reedplates.

Underside of the removed soundpost with the chord section

For a multi-tone sound by one key, several reeds must be supplied with wind simultaneously through one tone hole. This is made possible by the inner structure of the soundpost, which is divided into three separate chambers (black), each with 3 reed plates (red)...

Due to the bisonoric tone principle, 6 three-part chords can be called up on this soundpost. On the instrument a total of 12. Due to the high constructional effort, the chord range remains limited despite the bisonoric principle.

The high construction effort and space requirements for chord formation using individual chambers also explains why the bass side, in contrast to the melody side, remained bisonoric for a long time.

Only the double utilisation of a chamber with two chords made it possible to accommodate the most necessary chords. This is exactly what the bisonoric principle had once been conceived.

Only the later innovation of forming chords not by chambers, but by mechanismal coupling of single notes, made it possible to change over to the unisonoric principle and a larger number of retrievable chords.