Device (1) for displacing bones of a living mammal, including at least two engagement elements (2′7 2″), positioned at a distance from each other, for engagement with two separate bones, wherein each one of said engagement elements (2′, 2″) is carried by a support part (3′, 3″) which in turn is supported in a support portion (4′, 4″) of a distance unit (5), said support part and support portion forming a support unit, wherein each engagement element (2′, 2″) extends at an angle (α) with respect to a straight line (L) between two support units, and wherein at least one of said support units includes an arrangement for altering said angle. Said arrangement for altering said angle includes an actuating unit (V, 7″, 9′, 9″, 10′, 10″) which is maneuverable for adjustment of said angle (α). The invention also concerns a system for engagement with vertebrae including such a device and a use of the device.