A rotatable cutting tool, which extends along a tool axis defining an axial direction, includes a cutting formation in a first axial longitudinal end region, which is configured for material ablation on a workpiece, and includes a screw-connection formation in a second axial longitudinal end region opposing the first, which is configured for screw connection with a coupling unit. The cutting tool includes key-contact surface formations diametrically opposing each other with respect to the tool axis, which are configured for torque-transmitting abutting engagement with key surfaces of a key. Each of the key-contact-surface formations includes at least two key-contact surface regions provided with spacing from each other in the circumferential direction about the tool axis, which key-contact surface regions are inclined away from each other such that they form a vertex projecting away from the tool axis.