The system of the twelve modes – the “modal system” — provides useful schemes for the analysis of tonal structures of sixteenth-century polyphony, while the conventional “tonal system” of the 24 keys (tonalities) does so for the music of the eighteenth and nineteenth centuries. But what about the music of the seventeenth century? Should we approach this form the point of view from the modal system or from the tonal system? Both are possible and it is no news to say that seventeenth-century music operates in a transition area between the modal and the tonal system. But the two systems have basically different starting points, are how is a transition from the one to the other system possible? I my lecture I will propose a paradigm to describe this transition and to illustrate the transition with examples from musical repertoires and theoretical writings.