This music may be performed with any sound source (instruments, human voice, any kind of object, generators, etc.) suitable for producing the pitch of one of the four voices with a differentiation required by the piece. The four pitches are to be read in the treble clef, and only in the given register. Ali the sound sources may be modulated electronically. ln case of any kind of modulation the proportion of the modulated/ direct tone has to be selected in such a way that the constancy of the four tones should obvious at every moment of the performance. The sound sources used by the individual performers may also be completely homogeneous (eg. string ensemble), as far aspossible, however, the fewer the number of performers the more they should differ from each other. One performer should use no more than two to three sound sources, and at least one of these should be suitable for performing all four tones Instructions for the performance can be red in the preface of the score.