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Musical applications are very demanding on the expressive power of the underlined programming model. We propose PiCO, a calculus integrating concurrent objects and constraints as a base for music composition tools. In contrast with calculi such as JM95, RMW92] or TyCO Vas94], both constraints and objects are primitive notions in PiCO. The object model is extended with constraints by orthogonally adding the notion of constraint system found in the-calculus Smo94a]. The extended calculus provides a natural way to express more sophisticated communication behaviors via the standard message-passing synchronization mechanism. Moreover, it allows us to represent complex partially de ned objects such as musical structures in a compact way. The paper includes encodings in PiCO of the notions of class and subclass. We illustrate the transparent interaction of constraints and objects by a musical example involving harmonic and temporal relations.