The system of the anterior (a) and posterior (p) cruciate ligaments and their distances between points of attachment to the femur (f) and tibia (t), respectively, as found in the knee joint of tetrapods is considered as a crossed planar four-bar linkage. The maximum lower leg rotation range follows from the lengths of the bars of the cruciate ligament system. The extreme flexion and extension positions of the leg can be derived when this range is combined with the angles between the femoral and tibial long axes and the cruciate ligament system. The "retroposition" of the femoral and tibial articulating surfaces is determined by the dimensions of the cruciate ligament four-bar system and the bending range of the leg. The above theory is applied to data from the knee joints of ten species of mammals and a bird.