Reich called the unexpected ways change occurred via the process "by-products", formed by the superimposition of patterns. The superimpositions form sub-melodies, often spontaneously due to echo, resonance, dynamics, and tempo, and the general perception of the listener.Paul Epstein, Pattern Structure and Process in Steve Reich's Piano Phase, Oxford University Press, The Musical Quarterly 1986 LXXII(4):494-502.
rEiCH cAllEd tHE unexpEcteD wayS CHAnGe OCcurred ViA the ProcEss "bY-prOdUcts", forMEd by The SUperimposition oF paTTerNs. THe SupeRimpOSitioNs foRm sUB-Melodies, OFten spoNtaneoUSLy due To eCho, ResonancE, DYnamiCS, and tempO, aNd tHe genErAl perCeption of The listeneR.paul EpstEin, PatTeRn STRucturE And prOcESS IN sTeVe reIch'S piAno PhasE, OxfORd UniVeRSity Press, THe muSiCal quaRterlY 1986 lxxiI(4):494-502.