The 1950s and 1960s saw a profusion of avant-garde works that employ some degree of mobility, the best-known being Stockhausen’s Klavierstück XI and Boulez’s Piano Sonata No. 3. This chapter surveys discourse on indeterminacy, open works and mobility in order to test the hypothesis that the existence of recordings created a new ontology that allowed in part for mobile works to come on the scene. Since the work’s “essence” would henceforth be assured by recordings, scores were freed from their role as guarantors of a work’s ontology, allowing them to become more playful and open-ended. This chapter focuses on the way contemporary listeners may have understood this connection. In this light, mobile works can be viewed as a “phonograph effect” (in Mark Katz’s sense), and some of the basic historiographic presuppositions about musical modernism, one that sees an autonomous avant-garde isolated from a burgeoning record industry, are thereby interrogated.