Motion of the trapeziometacarpal joint was studied in 12 hands from fresh human cadavera. By use of a magnetic tracking system, a full range of motion of the first metacarpal was analyzed with respect to a defined trapezial coordinate system. The traces of the reference points on the head and base of the first metacarpal were monitored, and the instantaneous centers of rotation were calculated. During circumduction, the reference points on the head and base followed elliptical paths but in opposite directions. The average instantaneous center of circumduction was at approximately the center of the trapezial joint surface. In flexion-extension, the axis of rotation was located within the trapezium, and the path of the head was identical to the path of the base. In abduction-adduction, the axis of rotation was located distal to the trapezium within the base of the first metacarpal, and the base and head moved in opposite directions. There was no single center of rotation; rather, instantaneous motion occurred reciprocally between these centers of rotation within the trapezium and metacarpal base in the normal thumb. This changing instantaneous center of rotation results in a unique pattern of motion which is related to congruent, tightly constrained joint surfaces of two reciprocal saddle joints and to precisely positioned extraarticular ligaments.