By explaining how an Early Music orchestra produces its sound, we can review Howard Becker's concept of a convention. An orchestra's sound depends on principles incorporated in things (musical instruments, scores) and bodies (musicians' techniques). A common set of principles about interpreting a piece of music — principles acquired well before any rehearsal — do not suffice for coordinating a group of musicians. As observations have shown, face-to-face interactions are decisive in this coordination. The conductor is not omniscient and does not impose his interpretation on musicians. Relations based on authority, being unstable, are redistributed among the conductor, soloist and first violin during rehearsals. Recognizing the importance of face-to-face interactions draws attention to the cogency of power relations, which, though omnipresent, are constantly reworked in the situation for producing an orchestra's sound.