1. A device has been developed to measure talar tilt in both ankles simultaneously in comparable positions with equal stress on each.

2. An accurate and reproducible method for measuring the talar tilt has been used on 100 male subjects (200 ankles). These included 152 normal ankles, thirty-eight ankles with old or recent sprains, and ten ankles with other abnormalities.

3. Among the normal ankles 56 per cent had talar tilts ranging from 3 to 23 degrees, whereas among the sprained ankles 74 per cent had talar tilts ranging from 3 to 17 degrees.

4. In the fifty-nine patients with both ankles normal, the difference in tilt between the two sides ranged from 3 to 19 degrees in 22 per cent.

5. In eighty-eight extremities the amount of external rotation of the tibia caused by supination of the heel was found to vary from 0 to 22 degrees, with an average of 9 degrees.

6. The influence of the axes of motion of the peritalar joints and of the ankle joint on the function of the fibular collateral ligaments has been reviewed and analyzed.

7. By the use of models it has been shown that the external rotation of the segment composed of the tibia, fibula, and talus, which accompanies supination of the heel, is due to the deviation of the peritalar axis from the horizontal plane as distinct from its deviation from the sagittal plane. Also demonstrated by the models was the influence of each of the directional components of the peritalar axis of motion on peritalar motion.