This trihedron, Schaeffer's last piece, echoes the physicists "reference trihedron" linked to the three fundamental measurements of sound: frequency, duration and intensity. The basic signs of traditional sol-fa that enable to transcribe pitches, rhythms and nuances also correspond to these three measurements. However, it is precisely outside or beyond these parameters that, all through his life, Schaeffer researched music. Hence, qualifying this trihedron as fertile, is the confession "of a late repentance" On the other hand, and against all odds, this piece was only composed from synthetic sounds, developed by Bernard Dürr. It is Pierre Schaeffer's only purely electronic music work. Here, the synthesizer is rather monitored by sensitivity than by computation principles, thus letting uncanny structures emerge.