Its narrow shaft connects its widened base and rounded head; the former consisting of a thick cortical bone surrounding the open medullary canal; the latter two consisting of cancellous bone surrounded by a thin cortical shell.

The distal articular surface is quadrilateral, wide, and flat; thicker and broader transversely and extends much further palmarly than dorsally.

The palmar and medial surface exhibits a blunt ridge which separates a larger lateral part – the insertion of the opponens pollicis muscle – and a smaller medial part – the origin of the lateral head of the first dorsal interosseous muscle.

The configuration of the thumb carpometacarpal joint plays an important role in the mechanism of opposition.

On the palmar and lateral side of the base is the insertion of the tendon of the abductor pollicis longus muscle, usually featuring a small tubercle.