The music found in Robert ap Huw’s manuscript is not in the polyphonic or many-voiced style we might expect from high-art music in late-medieval Europe. Instead, the octave is divided into two sets of notes. One set, often G, C, D and g is designated the strong or sovereign set. The notes which come between – A, B, E and F – would then be the weak set. The strong set is so called because the notes are seldom re-tuned to other notes. The notes in the weak set are often re-tuned to different notes, by a semitone, tone or more to make different scales and modes. There were five established and warranted tunings and many more besides. The strong set is represented with a ‘I’, the weak set with an ‘O’.
The strong set corresponded to the stable and eternal parts of the universe and the weak set to the unstable and temporal parts of the universe as it was understood in the middle ages. The universe was imagined as a set of nesting crystalline spheres. The sun, moon and each of the planets (then known) was embedded in the thickness of a sphere. The earth formed the centre of this universe and around the spheres was arranged the firmament of fixed of fixed stars.
The music was composed using measures i.e. patterns of ‘I’s and ‘O’s e.g. Korffiniwr IIOO IOII or Mak y mwn hir IIII OOOO IOIO IIII OOOO IOII. During a measure one of the two sets predominates but notes from the other set may be recruited to make musical figuration. The measures, two of which are shown above, were used to generate new compositions and to memorise and classify pieces of music.
Through music composed on this basis the interplay of the eternal and the temporal, the stable and the unstable was made audible.