The thumb is a mechanical unit consisting of strong ligaments and dynamic muscles, providing the resistance to the fingers and palm during pinch and grip. Each thumb joint has a mechanical equivalent that helps explain elements of thumb stability. The trapeziometacarpal joint is potentially the most unstable but has adapted to its unique position by special joint geometry and, in particular, a combination of support ligaments that permit a wide range of motion yet stability. New anatomic and mechanical studies demonstrate the key support structures that must be duplicated by reconstructive surgical procedures if thumb stability is to be maintained.