Abstract: A switch knob has a mobile first part adapted to switch from one connection to another by rotation and a second part fixed to a support of the device on which the knob is mounted by an assembly device and a clamping device. The assembly device comprises two prism-shape members linked by a ring in two parts each including a hinge and is disposed between the second part and the support in a housing of the support provided for this purpose. The top of the prism-shape members faces towards the interior of the housing and their bases outside the housing so that clamping of the clamping device, by applying a force to the bases of the prism-shape members, moves the members towards each other and locks the second part to the support. Movement of the prism-shape members towards each other is facilitated by the hinges of the ring.