Organistrum: its tuning and keyboard

Organistrum: its tuning and keyboard



The aim of the present article is to present the ultimate result of my research on the possibility of rebuilding the Organistrum (Giuseppe Antonio Severini, Organistrum. Un caso di archeologia sperimentale. Tipheret, 2020). Italian text only.

1.Sources and features

The information we have on the instrument comes from a dozen images in books and works of art of the 12th century.

Constant elements

  1. The Organistrum has an « 8 » shape, with a rectangular extension.
  2. The dimensions are such as to occupy the knees of two men seated side by side.
  3. The wheel and the crank are a characteristic element, as are the three strings, in one case two, in one other perhaps one.
  4. The row of keys ends within the middle of the string.

Variable elements

 The number of keys can change: 6 to 11 (or 12).


Unknown elements

 I. There is no information about the tuning and the musical scale but from a plate in Martin Gerbert, De cantu et musica sacra, 1774. The same drawing gives an idea of the possible mechanism of the keys.

II. We know nothing more about the inner structure of the keyboard.


From these premises it follows that the replicas of the Organistrum so far attempted are at least partly hypothetical.

This obviously concerns the Variable elements  and the Unknown Elements.

Almost all the replicas take as a model the Organistrum of the Portico de la Gloria of Santiago de Compostela because, compared to all the other representations it is the richest in details, it has the maximum number of keys, it occupies the most important location.


There are 12 keys within the octave, the first of which should be the nut : they are definetly too many, unless you want to conclude that this is a chromatic keyboard.

Another arbitrary element lies in the choice of the generally adopted fifth and octave tuning, inspired by the organum parallelum polyphonic technique, described in texts that precede any known depiction of the Organistrum by at least a century or two. Instruments made following these hypotheses have little use in the performance of medieval music.


2. A different solution

In search of a different solution, I found myself reconsidering two objects that are certainly of the first level.


A.) The only drawing of the keyboard that we have left, complete with indications on the scale and on the structure of the keys, is the one copied by Martin Gerbert from a 12th c. manuscript, now lost, in Saint Blaise monastery.

Gerbert's keys have been interpreted as revolving bars with a long "spatula" tangent placed under the strings. The tangent would be brought into contact with all three strings simultaneously through a rotation. This system does not work in practice.

I prefer another interpretation of this drawing: the eight keys would be rotatable or lever acting from above the strings, pressing them against fixed frets, and it works.

Gerbert provides a scale for the instrument: from C to the next C with a single accidental, Bb. The fact that the intonations of the other two strings are not indicated leads to the conclusion that :

  1. they all had the same frequency;
  2. were in C with different octaves;
  3. Gerbert has forgotten to copy them.

The indicated scale is the canonical one and logically must be respected. The indication for the tuning being incomplete, this does not mean that it should be overridden. It is preferable to stick to hypotheses 1. and 2.

So we are faced with a diatonic instrument capable of producing all the intervals of the diatonic scale with a continuous sound. As several authors have observed, it seems to be an evolution of the Monochord. What could this instrument be used for? To provide a certain reference of the notes (a tuner), to play pedals in polyphony helping the singers in intonation.


B.) The musical repertoire coeval with the representations of the Organistrum, all of the 12th century, gives us more suggestions.

The manuscripts of Limoges, Winchester, Santiago de Compostela and Paris (Notre Dame), show an interesting two parts polyphonic repertoire. The scales are strictly diatonic. The only alteration used is Bb. These compositions were sung and intended for the liturgy. Many of these, called organum floridum or melismaticum, consisted of a bass vox principalis, and a higher vox organalis. Each note of the vox principalis corresponds to groups of many notes of the vox organalis. The extension of the vox principalis in the Magnus Liber organi of Notre Dame is mostly from C3 to D4, rarely up to F4. The compositions in the F key start at A2 and jump to C3, never requiring B2 and Bb2.

It is possible that the Organistrum as described by Gerbert effectively entered into the performance of these parts.


3.More than eight keys.

How should we consider keyboards with more than eight keys?

They are actually only two:

  • organistrum from Collegiata de Toro (Spain) : the nut + 9 keys ;
  • the one from Santiago de Compostela : the nut + 11 keys.

I think the most realistic keyboard is that of Toro, which only adds one tone (D) to the octave.

In the case of Compostela it is very difficult to think of an extension of the diatonic keyboard by two tones and a semitone without visibly exceeding half of the diapason. The Santiago instrument, certainly the most beautiful, has preponderant symbolic value, as amply shown in my book. The 12 keys do not want to indicate the intervals of the scale of the instrument, but those of the cosmic harmony according to Pliny, as can be seen listed next to many grids of planetary latitudes in 11th and 12th century manuscripts of astronomical content. The setting of the instrument at the center and at the top of the Portico describing the Glory of Heaven, the decorative details and the fundamental measures of the instrument have an evident astronomical and theological inspiration. Finally, the sculpture belongs to a masterpiece of the highest level, as one of the three most important sanctuaries of Christianity deserved.



4.Acoustic results

As with all theories, a crucial aspect lies in the experimental verification.

I have listened to many instruments, others I have heard in recordings, I built four different types myself. My conclusion is that the most coherent and most convincing version, with the most pleasant sound, is the one with the three strings in C3 and a diatonic fingerboard with levers acting over the strings.


Watch the videos:






On 2018, July 12th the famous Gate of Glory in Santiago de Compostela has been re-opened. This extraordinary document of art and thinking still leaves us astonished and admired. The many colours of the original gate have been partially restored, giving a new look,a new surprising light to the monument.

At the same time NEW researches have revealed that a polyphonic experience lies behind the depictions of musical instruments in the hands of the 24 Elders of the Apocalypse, all around the arch.

All instruments are depicted in couples (2 harps, 2 lauds ...etc.) suggesting they can be played in duet. The so-called Organistrum, at the top of the arch, played by two musicians,  might be showing a chromatic scale. Does this fact indicate an extraordinary evolution in musical thinking at Santiago de Compostela during the 12th century ?

Furthermore, two-voices compositions in Codex calixtinus witness the developement of Polyphony at Santiago in the same age.

In our studies we have demonstrated that Portico de la Gloria top instrument is a SPECIAL ONE. In fact, its scale, apparently chromatic -useless for sacred music at that age - could be simply diatonic, but useful to perform the vox principalis in XII century polyphonies like those of Codex Calixtinus.

This shed a new light over the whole Portico della Gloria music instruments setting: the couples are actually showing a polyphonic idea, not following only a sort of scenical arrangement.




The book: 





polyphonic keyboard

Organistrum: new polyphonic keyboard

My reconstruction of the instrument is mainly based on my own interpretation of its possible musical function. Since I want to consider it  as a genuine polyphonic instrument I  invented  a  special  keyboard, fit for 12th century two voices polyphonies, suggesting also a different managing of the wheel.


By tuning the strings  either:  A – d – a   or:  A – e – a    we get an overall extension of two chromatic octaves (minus the last semitone). Before lifting each key, the performer  can choose which string he is going to touch, simply by turning the key to the proper position: the first one allows him to operate on the bass string, the second on the middle one, the third on the higher string.

Thus it is possible to play two different melodic lines simultaneously. In Santiago sculpture the hands of the musician on the right are on the third and on the fifth key. This means he is playing  -c, g -  rather than  -d, f -  bichord on bass and middle strings,  or  - g, c’  rather than  - f, d’- bichord on middle and higher strings (first tuning).


First string                              a..       a#      b     c’     c’#     d’

Middle string                         d..       d#      e      f      f#      g    

Bass string                            A..       A#      B     c     c#      d

Keys                                      0          1       2     3       4      5


 In position 4 the key is provided with two tangents so as to act on  second and  third strings (tuned either in Fifth or Fourth)  at the same time to perform  Organum parallelum, leaving the bass string as a drone. 

How they calculated the semitones progression

Known medieval texts neither describe  nor discuss about the possibility of a chromatic scale. But, reading Boethius's De institutione musica, easy to be found in monastic libraries since 9th century on, the monks could learn a lot about semitones both from pythagorean tradition and from Aristoxen's. There are only few lines, but enough, dedicated to the description of Aristoxen's practical method to divide the monochord. From this source the medieval scholar could learn a practical method in order to draw a correct chromatic scale of 12  tempered semitones. We discuss about this in another blog in this website

connecting astronomical and musical  knowledges through 10th and 12th centuries. Anyway, this idea comes from very early roman sources: Plinius and Boethius. In case the author of Santiago instrument intended to describe a revolutionary full chromatic/polyphonic keyboard we can suppose he was aware of that very ancient tradition.



On the other side, the musician who is in charge of turning the crank, by a smooth, even movement of his right hand,  can also lift one or even two of the strings from the edge of the wheel with his left hand in order to stop them vibrating. This way you can either avoid conflicts between the voices or stop  undesired drone effects.  









I chose one big  Red Willow (Salix purpurea) planck, seasoned in nature, from which I  carved the sound box and the base of the keyboard in one piece, average thickness  8-10 mm. Flat back, flat sides as in the original. 

GENERAL MEASURES:                      

 Total length    940mm                                                                                                           

Max width      230mm                                                                                                              

 Depth              80mm                                                                                                                

 Diapason        720mm


In the bottom of sound box I drilled a 10mm hole for the wooden axis, made of Beech (Fagus silvestris). Axis ends into another hole of 10mm drilled directly with no axle box in a wooden bar (Spruce) 15mm thick, glued between the two lobes of sound box which has been carved out of one piece of Spruce (Picea  abies) 8mm thick with no other bars glued underneath. Keyboard  box is independent from the body of the instrument and can be easily removed to modify  general level of the bars, changing their angle with the strings or making reparations. The 11 bars of the keys, diameter 10mm, are of Pine (Pinus nigra) and the 55 pivoting tangents of Beech. The problem of the distances among the bars is discussed in the present article. To avoid noise made by the bars returning to their previous position, after being pulled up to play, a stripe of cloth has been glued at the bottom of the keyboard. The carved lid has been made of Spruce, 8mm thick and is simply interlocked with the body of the keyboard without any hinge or other device.

The wheel, 11cm  wide and 20mm thick is of Walnut (Juglans regia) and forced on  wooden axis without any screw, nail, or wedge. No axle box at the end. The Beech crank , stuck in the square end of the axis, can be easily removed. The soundboard is glued to the body of the instrument , no sound pole in it. The bridge is of Poplar (Populus nigra) reinforced with a Beech edge. The tailpiece  of Chestnut (Castanea sativa) is linked to the bottom of sound box by a leather lace. Tuning pegs are of Beech, no need for a tuning key. Two light  layers of pure almond oil have  been used as  finishing . Gut strings: 0.80, 1.10, 1.40mm.






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