The system of the anterior (a) and posterior (p) cruciate ligaments and their distances between attachments to the femur (f) and tibia (t), respectively, as found in the knee joint of tetrapods is considered as a crossed four-bar linkage. Starting from a flat or curved tibial articulating surface the shape of the femoral articulating surfaces in principle can be derived (Huson, 1974; Menschik, 1974). The point of intersection (S) of the cruciate ligaments is the instantaneous centre of rotation and describes a curve during knee angulation; the centrode. Two centrodes are distinguished: S1 with the tibia stationary and S2 with the femur stationary. During leg bending the centrodes roll over each other, without sliding. Four series of knee-joint simulations were made. In series A, the femoral bar (distance between cruciate ligaments) was varied, in series B, the anterior cruciate ligament (so the length of a ligament). In series C and D, combinations of bar lengths were varied. The maximum leg rotation range was calculated from the lengths of the bars of the cruciate ligament system for each of the series A ... D. Also the leg rotation range for which the knee joint is mechanically stabilized by the cruciate ligaments was calculated. The stabilization criterium chosen was that a cruciate ligament may not become perpendicular (> 78.5 degrees) to the articulating surfaces. It was found that the cruciate ligaments alone cannot stabilize the knee joint adequately over the whole required range of leg movement. External structures are required to obtain full stabilization. As the femoral and tibial articulating surfaces never lie at the centrodes, considerable sliding occurs between them. It is suggested that a uniformly distributed sliding is essential for lubrication and so for a proper knee-joint functioning.