Experiences gained from the "Studies" made plain that it was an unacceptable oversimplification to regard timbres as stable entities . Reinforced by his studies with Meyer-Eppler, beginning in 1955, Stockhausen formulated new "statistical" criteria for composition, focussing attention on the aleatoric, directional tendencies of sound movement, "the change from one state to another, with or without returning motion, as opposed to a fixed state" . Stockhausen later wrote, describing this period in his compositional work, "The first revolution occurred from 1952/53 as "musique concrète", "electronic tape music", and "space music", entailing composition with transformers, generators, modulators, magnetophones, etc; the integration of all concrete and abstract (synthetic) sound possibilities (also all noises), and the controlled projection of sound in space" (, reprinted in ). His position as "the leading German composer of his generation" was established with "Gesang der Jünglinge" and three concurrently composed pieces in different media: "Zeitmaße" for five woodwinds, "Gruppen" for three orchestras, and "Klavierstück XI" . The principles underlying the latter three compositions are presented in Stockhausen's best-known theoretical article, ". . . wie die Zeit vergeht . . ." (". . . How Time Passes . . ."), first published in 1957 in vol. 3 of "Die Reihe" . 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" . 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 . Still other works permit different routes through the constituent parts. Stockhausen called both of these possibilities "polyvalent form" , 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) .