My lecture considers the performative dimension of counterpoint as it was practiced within the choirs and chapels of many European churches in the 15th and 16th centuries. After discussing its status in the current literature, I would like to embark on a three-stage inquiry : firstly, I will give a quick overview of the non-musical documents that inform us about the circumstances of the teaching and performance of the chant sur le livre. I will turn in a second time towards the theoretical literature : how was it taught, according to which techniques ? Finally, I will briefly investigate a written out repertoire that has been mostly neglected so far : we have preserved a number of pieces that aim to imitate the sound and texture of contrapuntal improvisations, and studying this material allows us to get a more precise idea of how the contrappunto alla mente, as it was called in Italy, actually looked like when properly done.