FIG. 27 shows the simultaneous positions of both legs during a step, divided into four groups. The first group (_A_), 4 to 7, gives the different positions which the legs simultaneously assume while both are on the ground; the second group (_B_), 8 to 11, shows the various positions of both legs at the time when the posterior leg is elevated from the ground, but behind the supported one; the third group (_C_), 12 to 14, shows the positions which the legs assume when the swinging leg overtakes the standing one; and the fourth group (_D_), 1 to 3, the positions during the time when the swinging leg is propelled in advance of the resting one. The letters _a_, _b_, and _c_ indicate the angles formed by the bones of the right leg when engaged in making a step. The letters _m_, _n_, and _o_, the positions assumed by the right foot when the trunk is rolling over it. _g_ Shows the rotating forward of the trunk upon the left foot (_f_) as an axis. _h_ Shows the rotating forward of the left leg and foot upon the trunk (_a_) as an axis. Compare with fig. 4, p. 21; with fig. 24, p. 47; and with fig. 26, p. 55.--After Weber.] The alternate rotation of the trunk upon the limb and the limb upon the trunk is well seen in fig. 27, p. 59. At _A_ of fig. 27 the trunk (_g_) is observed rotating on the left foot (_f_). At _D_ of fig. the left leg (_h_) is seen rotating on the trunk (_a_, _i_): these, as explained, are complementary movements. At _A_ of fig. the right foot (_c_) is firmly placed on the ground, the left foot (_f_) being in the act of leaving it. The right side of the trunk is on a lower level than the left, which is being elevated, and in the act of rolling over the foot. At _B_ of fig. the right foot (_m_) is still upon the ground, but the left foot having left it is in the act of swinging forward. At _C_ of fig. the heel of the right foot (_n_) is raised from the ground, and the left foot is in the act of passing the right. The right side of the trunk is now being elevated. At _D_ of fig. the heel of the right foot (_o_) is elevated as far as it can be, the toes of the left foot being depressed and ready to touch the ground. The right side of the trunk has now reached its highest level, and is in the act of rolling over the right foot. The left side of the trunk, on the contrary, is subsiding, and the left leg is swinging before the right one, preparatory to being deposited on the ground. From the foregoing it will be evident that the trunk and limbs have pendulum movements which are natural and peculiar to them, the extent of which depends upon the length of the parts. A tall man and a short man can consequently never walk in step if both walk naturally and according to inclination.[39]