The joint at the base of the thumb is a saddle joint, according to InnerBody. Saddle joints are a subclass of synovial joints in which one bone is curved like a saddle and another bone rests inside the curve like a rider.
Of the five carpometacarpal joints in the hand and wrist, the joint at the base of the thumb is the only saddle joint, explains InnerBody. This type of joint allows elliptic movement and enables the thumb to act in opposition to each of the other fingers. This opposable thumb provides firm gripping power and allows the use of tools.