For each of his compositions for prepared piano Cage created a specific piano preparation chart, setting out in meticulous detail the strings to be prepared, and the materials and manipulations to be used for preparing them. For the Concerto for Prepared Piano and Chamber Orchestra (1951), 53 tones of the keyboard must be prepared; and in this case Cage himself was astonished at the complexity of these preparations. The range of sounds is further expanded by an extra bridge installed in the piano, which enables the generation of microtones. In view of the charts and the rigorous treatment of the orchestra, the composition itself is very close to serialism; but already the specific sound events were difficult to predict with any precision due to the complex preparation of the piano. In Sixty-Eight (1992), which (like many of Cage's works) derives its name from the number of musicians, all dimensions of sound are left to the musicians' discretion, always subject to the temporal structure. The pitches (15 different tones) are given; the instruments are divided into voices.