he called the sound source a vibrator, and in later publications called the body that agitates the vibrator an inciter. This book was important because it was one of the first to discuss the methods and parameters of timbral organization. Though these were not new ideas, it is likely Penderecki used this information from Drobner to develop his own timbre system. Because Penderecki understood timbre primarily as a function of the materials, the timbral categories in his sonorism are based upon the most common materials used in the assembly of instruments and accessories of traditional symphonic orchestras: metal, wood, leather, felt, and hair. Though any of these materials can act as both vibrators and inciters, felt and hair are the least effective vibrators due to the fact that they would be inaudible when vibrated. This then means that at least one of the two sound-generating bodies must be made of metal, wood, or leather, referred to as primary materials. In Penderecki’s timbre system, the three primary materials may (and do) interact with one another, whereas felt and hair can only interact with one of the primary materials and not themselves. Though he was inspired by Drobner, Penderecki’s timbre system goes another step further, in which “it is of no importance whether metal, wood, and leather are represented by a vibrator or by an inciter, both colliding bodies being of equal weight as primary materials.” This means that “if a given body can be a sound source—that is, if it is made of one of the three materials capable of performing this function (m, w, l) --then it becomes a sound source regardless of whether it is hit, rubbed or plucked or itself hits, rubs or plucks.” Though Penderecki’s timbre system used material categories found in the traditional symphonic orchestra, he had to employ drastic changes to produce the desired effects. Because metal predominates typical orchestras (metal strings, metal percussion instruments, etc.), Penderecki needed to find ways of enlarging the representation of wood and leather. Though this was the case for other works, Polymorphia is based on an opposition of metal and wood. Since the piece is written solely for stringed instruments, Penderecki used a variety of methods to create opposing sections of metal and wood. Form The work follows a traditional ABA’ formal framework, with each section roughly three minutes. Penderecki delineates each section through a system of contrasting timbres.