Timbre often indexes an instrument’s materiality, and timbral variation often correlates with a player’s actions. Yet synthesizers complicate phenomenological links between sound and source. This chapter juxtaposes three instruments: an electromagnetic tuning-fork apparatus, developed by the nineteenth-century scientist Hermann von Helmholtz; the RCA Mark II, used by Milton Babbitt and other mid-twentieth-century composers; and the Yamaha GX-1, a large polyphonic synthesizer from the 1970s, played by Stevie Wonder and Keith Emerson. These synthesizers create new timbres and also imitate acoustic instruments, in a process that Robert Moog calls “timbral thievery.” Such imitations can provoke exaggerated or anxious discussions of synthetic and natural timbre. At the same time, performers may showcase the gap between timbre and instrument, exploiting a sense of uncanny or ambiguous sound sources for varied expressive ends. Ultimately, then, synthesizers help musicians both produce and conceptualize timbre.