This article describes an interactive notation paradigm, which aids in structuring flexible performances for an arbitrary number of participants and any combination of acoustic or electronic sources. A simple system allows a 'maestro' to organise an ensemble and to communicate information to the members by means of an interactive window projected on a surface visible to all (performers and audience). The following text describes the motivation and design of the notation strategy, its implementation in the SuperCollider environment and discusses some compositional, performative and pedagogical issues with reference to a recent work; in this context the 'system' is considered to be the 'piece' itself.