Improvisational practices are deeply linked to musical instruments. Indeed, by definition, to improvise is to create music with (or sometimes against) an instrument (including voice); in many ways, then, the act of improvisation is inseparable from the instrument onto which it is produced. As saxophonist Steve Lacy once said, “the instrument – that’s the matter – the stuff – your subject”(quoted in Bailey 1992): it plays a crucial role both in the improviser’s creative process over the course of the performance and in the construction of a singular artistic signature over her career. Arguably, this appropriation of the instrument that is consubstantial to improvisational practices is pushed even farther in the free improvisation scene, where it is not uncommon to meet with musicians who use their very own improvisation device – from custom-made instrumental preparations to the h(ij)acking of various electronic devices to the building from scratch of brand new instruments. Which processes underlie the creation of such musical devices dedicated to improvisational practices? What problems arise in their constitution? Which criteria do the improvisers use in selecting the various objects that make up their set? How do these custom-made instruments or devices allow them to address the specific constraints of free improvisation, and particularly of collective free improvisation? In order to answer these questions, I’ve interviewed 15 professional French improvisers (including pianists Ève Risser and Sophie Agnel, trombonist Thierry Madiot, tape- recorder-player Jérôme Noetinger, mixing-desk-player Arnaud Rivière, turntablist ErikM, guitarists Julien Desprez and Pascal Marzan, drummer Toma Gouband, “prepared chamber”-player Anton Mobin, “spat”-player Nicolas Chedmail, etc.), following each time the same protocol: I first video-taped them while performing a short improvisation with their instrument; I then asked them to watch the video and to comment out loud their musical and instrumental actions; finally, I asked them specific questions about their instrument, most notably regarding its evolutions over time. The many devices I’ve studied differ along six main dimensions: their degree of predictability; their degree of material diversity; their degree of autonomy; their degree of openness; their degree of immediacy; and their degree of functionalization. Indeed, in the creation of their device, musicians must deal with regulative objectives that often appear to be contradictory: the instrument must be controllable but it must also allow for some unpredictability; it must possess a large timbral range but it must also allow for quick reactivity; etc. Here, I will show how the different solutions that the musicians came up with to conciliate those regulative objectives reflect their conception of improvisation and the values that they ascribe to the practice of improvisation. Beyond a better understanding of the processes that underlie the creation of these improvisation devices, this presentation will also contribute to the material turn that has been recently promoted in musicology (De Souza 2017, Butler 2014), by shedding some light on the crucial role played by the material environment in the improvisers’ creative process: not only does the improviser’s instrument act as an “extended” mind (Clark and Chalmers 1998), a cognitive artefact carrying information that is exploited, manipulated and/or transformed as part of the improvisational process; it is also in itself an object of the improviser’s creativity – being progressively shaped and transformed under the improviser’s actions.