This famous image comes from Gerbert's pubblication entitled: De cantu et musica sacra, written in the 18th century.
The author copied a drawing from a 13th century manuscript, now lost. It is considered the most important written source regarding the instrument called Organistrum : it shows clearly its general shape, the number of strings, the presence of a cranck and a wheel, the keyboard and the musical scale.
Unfortunately, the information about the tuning is incomplete: only one string out of two is indicated: c.
May be the author judged this indication was enough (intelligenti pauca), so, we could consider two solutions:
- all three strings tuned in c,
- one in c and the other two "in harmony ", i.e. c, g, c'
Since the keys seem to operate the three strings at a time, we would be able to play a whole c-c' diatonic scale in the first case and a sequence of parallel fifths and octaves in the second one.
The last pattern has seduced many luthiers and musicians in the last decades, cheated by the erroneous attribution to Odo de Cluny (10th century) of a manuscript entitled "Quomodo organistrum construatur".Their idea was that the Organistrum - certainly devoted to sacred music - could have been related to the contemporary polyphonic technique called Organum parallelum .
But the famous text, once considered the earliest source for the studies about this instrument, has been recently recognised as a 13 th c. work by anonymous author.
Since the story of the instrument has been relocated in the right period, the 11th and 12th century, we observe that the musical scene looks rather different. Many anonimous composers were developping much more sophisticated and interesting polyphonic techniques: discantus, organum duplum and organum melismaticum or floridum , organum parallelum being no longer attested.
A new reason was then adduced by some authors to justify a parallel fifth and octave tuning: the misleading interpretation of the letters written around the instrument wheel in Gerbert's drawing: m a G/ a D a.
These letters could allude to one or more tuning patterns of the instrument: a D a, or d D a.
But it is rather difficult to explain the first part: m a G. Eventually, all interpretations based on such letters cannot match with a C scale.
In fact, the letters compose the word magada that means bridge. This word was used frequently in latin 11th and 12th century music treatises, to indicate the bridges of the Monochord. It comes from Ptolemeus Harmonikai and was transmitted by Boetius in De institutione musica, lib.IV, chapter 18, in his description of the monochord: "duo semisperia, quas magadas greci vocant". Thus, the monochord bridges were half spheres, as stated by Ptolemeus, in order to touch the string in one point only.
It is interesting to point out that 11th c. treatise entitled Divisiones monocordi, inc. Studiosis necdum in musica provectis hec de monocordo...
M 17 sup, (Biblioteca Ambrosiana, Milan ) still reports the name magada to indicate the monochord bridges, while this word is changed into rotulum in 13th c. manuscript Quomodo organistrum construatur, in which the word rotulum logically indicates the semispherical bridge and not a wheel (rota).
As a matter of fact all these texts and others called Mensura organistri are concerned with the description of the pythagorean diatonic C scale
and the method used to draw it by ruler and compass following simple ratios and they never describe any musical instrument but the monochord.
I would like to quote some lines from Francis W.Galpin, Old english instruments of music. 1910 (old books are not always the worse ones !):
"The Organistrum, for such was its name at first, is undoubtedly derived from the Monochord, a simple contrivance for ascertaining the intervals of the musical scale by a series of movable bridges".
In medieval Europe the C diatonic scale had an outstanding theorical value both because it offers the essential pattern of the Pythagorean system and because it contains the basic natural exachord of Guido d'Arezzo, but in sacred music practice the more frequently used modes were protus autenticus (D) and plagalis (A), protus autenticus offering also the musical scale generally associated to musica mundana or music of the spheres.
Then, even if I would love to say that the wheel instruments were invented to make the continuous "circular" sound of the heavens audible, I definitely can't.
First of all the C scale is not the planetary one, secondly the presence of b betrays a practical concern, and finally, you cannot hear all the sounds together.
So, I guess that, with all strings tuned in C, the Organistrum could have been useful in backing the tenor part in compositions called Organum melismaticum, like the Romanesque pull-organ, and unlike later noisy "country-folk" instruments.