 His work with electronic music and its utter fixity led him to explore modes of instrumental and vocal music in which performers' individual capabilities and the circumstances of a particular performance (e.g., hall acoustics) may determine certain aspects of a composition. He called this "variable form" (Wörner 1973, 101 – 105). In other cases, a work may be presented from a number of different perspectives. In Zyklus (1959), for example, he began using graphic notation for instrumental music. The score is written so that the performance can start on any page, and it may be read upside down, or from right to left, as the performer chooses (Stockhausen Texte, 2, 73 – 100). Still other works permit different routes through the constituent parts. Stockhausen called both of these possibilities "polyvalent form" (Stockhausen Texte, 1: 241 – 51), which may be either open form (essentially incomplete, pointing beyond its frame), as with Klavierstück XI (1956), or "closed form" (complete and self-contained) as with Momente (1962 – 64 / 69) (Kaletha 2004, 97 – 98).
