The clinical features which distinguish hypermobile flat-foot with short tendo achillis are: 1. A prolonged history of some degree of foot disability extending back into childhood, the degree of disability tending to increase. There is often an hereditary tendency. 2. Flat-foot deformity which is mobile. It disappears when the feet are freed of weight-bearing, and appears when the patient stands. It can be corrected by muscular effort. 3. A short tendo achillis, which limits dorsiflexion at the ankle joint. 4. Hypermobility of the mid-tarsal and subtalar joints. 5. A deformity which is often severe and of characteristic appearance. Evidence from anatomical and roentgenographic studies shows that hypermobile flat-foot with short tendo achillis results from unstable architecture of the tarsal bones, especially their configuration and position in relation to each other. This results in imperfect support of the head of the talus by the calcaneus.