The Bible of Stephen Harding

The text describes a slider organ depicted in the Bible of Stephen Harding, Abbot of Cîteaux, dated between the end of the eleventh and the beginning of the twelfth century. The illustration shows King David seated with the harp and sceptre, flanked by various musicians, including a simply drawn organist, seated and with his back bent. The instrument has a thick wind chest supported by two large oblique wind conductors serving as feet, connected to a conflatiorium receiving air from twelve bellows, although not all are visible in the drawing.
The wind chest supports two rows of eight conical pipes, with the treble section to the left of the organist. These pipes present quadrangular openings at mid-height, whose exact role is not clear; they are probably analogous to the slits of harmonic flutes, as recognised in some specimens in the Naples Museum. The keyboard is well depicted with eight slider keys, long and vertical, although this last characteristic is probably a perspectival error on the part of the artist.
An important detail is that each key is marked with a letter according to the directions of the Anonymous of Bern to help the organist. The scale depicted is diatonic with the addition of the Synemmenon, indicated by letters such as C, D, E, F, G, a, b, b natural. In the image, the organist is pressing key D and preparing to pull F, although the order of the letters does not correspond to that of the pipes, a sign of an inconsistency in the drawing.