occurs at only three different points (not counting the repetition in bar 3), each initiating a new and major formal subdivision of the piece." Timothy Kloth interprets the head motif or, "the molecular structure around which the entire composition is cast," as being the five-pitch-class motive which opens the piece, F–E–F–C–G. This is divided into two trichords: "cell X" (also Perle's head motif, F–E–F) and "cell Y", F–C–G. Recordings George Perle and Marc Wilkinson, in writing, and Harvey Sollberger, in recording, all interpret the fourth note in bar twenty-three as a B natural. The piece has been recorded and released on: Members of the Columbia Symphony Orchestra, directed by Robert Craft – Varèse: Density 21.5, Hyperprism, Intégrales, Ionisation, Octandre, Poème Electronique (1959) CBS Masterworks – CBS 60286. Severino Gazelloni, flute (1967) WERGO WER 60029, Series: Große Interpreten neuer Musik. Ensemble Instrumental de Musique Contemporaine de Paris, directed by Konstantin Simonovitch – Varese: Deserts / Hyperprism / Integrales / Density 21.5 (1971) La Voix De Son Maître 2 C 061-10875 Y. Edgard Varèse: The Complete Works (1994/1998). Performed by Jacques Zoon. Decca London Polygram 289 460 208–2. Assistance and advised from Professor Chou Wen-chung, who worked closely with Varèse. Harvey Sollberger Philippe Bernold, Alexandre Tharaud, The Solo Flute in the 20th Century (2008), Harmonia Mundi Musique d'Abord HMA1951710 Claire Chase, Density (2013) New Focus Recordings Notes and references Notes References Sources Further reading Babbitt, Milton (1966). "Edgard Varèse: A Few Observations of His Music". Perspectives of New Music 4, No. 2 (Spring–Summer): 14–22. Bernard, Jonathan W. (1981). "Pitch/Register in the Music of Edgard Varèse". Music Theory Spectrum 3 (Spring): 1–25. Bernard, Jonathan W. (1986). "On Density 21.5: A Response to Nattiez". Music Analysis 5, nos. 2–3 (July–October): 207–231. Brouwer, Candace (1997–98). "Pathway, Blockage, and Containment in Density 21.5". Theory and Practice 22–23:35–54. Guck, Marion (1984). "A Flow of Energy: Density 21.5". Perspectives of New Music 23, no. 1 (Autumn–Winter): 334–347. Kresky, Geoffrey (1984). "A Path Through Density". Perspectives of New Music 23, no. 1 (Autumn–Winter): 318–333. Marvin, Elizabeth West (1991). "The Perception of Rhythm in Non-Tonal Music: Rhythmic Contours in the Music of Edgard Varèse". Music Theory Spectrum 13, no. 1 (Spring): 61–78. Nattiez, Jean-Jacques (1982). "Varèse's Density 21.5: A Study in Semiological Analysis", translated by Anna Barry. Music Analysis 1, no. 3 (October): 243–340. Siddons, James (1984). "On the Nature of Melody in Varèse's Density 21.5". Perspectives of New Music