The word organistrum is derived from organum and instrumentum; the former term was applied to the primitive harmonies, consisting of octaves accompanied by fourths or fifths, first practised by Hucbald in the 10th century. This explanation enables us to fix with tolerable certainty the date of the invention of the organistrum, at the end of the 10th or beginning of the 11th century, and also to understand the construction of the instrument.
A stringed instrument of the period — such as a guitar fiddle, a rotta or oval vielle — being used as a model, the proportions were increased for the convenience of holding the instrument and of dividing the performance between two persons. Inside the body was the wheel, having a tire of leather well rosined, and working easily through an aperture in the soundboard. The three strings resting on the wheel and supported besides on a bridge of the same height all sounded at once as the wheel revolved, and in the earliest examples the wooden tangents taking the place of fingers on the frets of the neck acted upon all three strings at once, thus producing the harmony known as organum.
The organistrum comes as a shared instrument, and thanks to the combined action of two people can reproduce a simple but fundamental harmonic base. Thanks to this property it became the leading instrument of the sacred music, up until then guided just by the voices. The musical compositions of that time became then stimulated and supported by this new perspective, and this led to the development of new architectures, which started having more and more elaborate shapes.
Each of the two new musicians has a new task that refers to different levels of expertise; one has to spin the crank wheel to generate the vibration of multiple strings at the same time, so they can generate a multiple continuous sounds; the other one presses the mechanical buttons which produce different sounds according to their length.
Together they can be complementary and functional, and this synergy creates a harmonic base which tends to the musical beauty, opening new possible scenario, thus projecting the XII century musical reality to a whole new level.