[4] In 1948 and 1949, Boulez worked on Livre pour quatuor for string quartet, in which he began to explore the notion of expanding serial technique to encompass rhythms and dynamics as well as pitches.
[5] This was followed by further efforts in the direction of integral serialism in the form of the first book of Structures for two pianos, Polyphonie X, and Deux Études for tape, as well as an article titled "Eventuellement"[6] containing detailed comments and reflections on the experience of composing the pieces.
[7] (Paul Griffiths noted that this concept represented an early example of what would later become known as an "open" work, in that it would have allowed the conductor to freely select a subset of pieces.
[8]) In the remainder of the letter, Boulez laid out his thoughts with regard to the serial treatment of pitch (including quarter tones), rhythmic cells, and instrumental combinations (and thus timbre).
[10]) In the winter of 1950–1951, Heinrich Strobel visited Boulez and found him wrapped in a blanket, surrounded by charts and score pages covered with tiny notes.
[24] He attended the concert, and eventually Stravinsky and Boulez met and formed a relationship which would run "hot and cold" until the older composer's death.
[25] Reflecting on his experience with Polyphonie X, Boulez wrote the following in a letter to Henri Pousseur dated late 1952: "I fear I let myself go a little too much, presently, in terms of virtuosity of pointillist technique, without referring, strictly speaking, to overall compositional sensibility.
[30] In 1954, he published an essay ("Recherches maintenant") that was both self-critical and forward-looking, in which he lamented the monotony of recent serial works, and in which he began to express a concern with perception, suggesting that his path to the future lay in seeking a balance between rigor and free will.