Free improvisation is often presented as a form of musical creation where preliminary decisions or preexisting plans are kept to a strict minimum. However, long-standing groups and collaborations that span over many years are not uncommon in the free improvisation scene. One might wonder, then, how do these musicians work together? How do they manage to balance the openness, spontaneity, and unpredictability of free improvisation with the unstoppable normalizing force of familiarity? In order to answer these questions, we need to understand what is at stake during rehearsals of free improvisers. What do these improvisers do when they work and practice together, since they literally have nothing pre-established to rehearse, or at least no pre-composed material, such as standards, arrangements, chord charts, and themes? This paper presents the results of an ethnographic study conducted with three Paris-based improvisation ensembles: the Orchestre des Nouvelles Créations, Expérimentations et Improvisations (“Orchestra of new creations, experimentations, and improvisations”); a duo comprising pianist Ève Risser and clarinetist Joris Rühl; and a quintet made up of the five founding members of the “Umlaut” collective. This data is used to show the different functions rehearsals have for these improvisers.