Abstract: The key has a head (2) provided with a window (3) enabling to see a portion (5) of the surface of the cylinder of the body (4) of the key. Two grooves (16) extend each one over one half of the circumference of the body (4), they are mutually offset by 180 degrees and ended by bores (7). In the service position, pins (12) are urged towards the bottom of the bores (7) by a resilient member (13). When the torque applied to the head (2) of the key increases, the pins (12) are expelled from the bores (7) and the head of the key can rotate by 180 degrees relatively to its shank (1), thus showing the opposite portion of the body (4) through the window (3).