There are two possible versions of Mutationes:
1: For one player. In this case the first movement is repeated as the seventh and last movement in order to provide a framework for the cycle. If two organs – playable from one console – are not available, then the even-numbered movements (II, IV, VI) should be performed in a registration which imitates a small Baroque organ.
2: For two players on two organs. The two soloists having played the first six movements alternately, the ‘Duo’ is performed together as a finale. The registration of this movement will depend on the tonal balance of the two instruments, but obviously the natural difference between the large and the small organ must be preserved.
From a formal point of view the cycle is a centrally-planned structure. In both versions movements I and VII provide a clear framework, while the true centre of gravity lies in the golden section, in movement V.
from notes by Petr Eben © 2002
English: Julia Rushworth and Atlas Translations