This paper presents the musical interactions aspects of the design anddevelopment of a web-based interactive music collaboration system called JamOn. Following a design science approach, this system is being built accordingto principles taken from usability engineering and human computer interaction(HCI). The goal of the system is to allow people with no to little musicalbackground to play a song collaboratively. The musicians control the musicalcontent and structure of the song thanks to an interface relying on the freeinking metaphor. One contribution of this interface is that it displays musicalpatterns of different lengths in the same space. The design of Jam On is basedon a list of performance criteria aimed at ensuring the musicality of theperformance and the interactivity of the technical system. The paper comparestwo alternative interfaces used for the system and explores the various stagesof the design process aimed at making the system as musical and interactive aspossible.