Referring back to the cardboard model, notice that when the cardboard is allowed to untwist—by inclining the vertical end in one direction and keeping the other end flat on the table—the arch flattens. Inclining the vertical end in the opposite direction increases the twist and increases the arch. In the foot, inclination of the vertical component of the structure, the calcaneus, will result in similar untwisting or twisting; this results in a respective decrease or increase in the arching of the foot, if the metatarsal heads remain in contact with the ground (Fig. 16-26). The person who stands with the heel in a valgus position will have a relatively flat or untwisted foot, whereas a person whose heel is in a varusposition when standing will appear to have a high arch because of increased twisting between hindfoot and forefoot. The former situation is often termed a pronated foot or flatfoot, and the latter is termed a supinated foot or pes cavus. In the situation of the heel remaining in a vertical position but the metatarsal heads are inclined, as on an uneven surface, the effect will also be to twist or untwist the foot, thereby raising or lowering the arch. For example, if the inclination is such that the first metatarsal head is on a higher level than the fifth, the forefoot supinates on the hindfoot, untwisting the foot and lowering the arch. Note that supination of the forefoot with the hindfoot fixed is the same as pronation of the hindfoot with the forefoot fixed; they both involve untwisting of the tarsal skeleton from motion at the subtalar, transverse tarsal, and tarsometatarsal joints.