In contrast to the metacarpals 2-5, the first metacarpal is embryologically of phalangeal origin (equivalent to the proximal phalanges of the other digits). This term applies if the first metacarpal is partially duplicated. Depending on the severity the appearance on x-ray can vary from a notched metacarpal (the duplicated bone is almost completely fused with the metacarpal) to a partially fused appearance of the two bones. In most cases the fused bone resembles a Y-shape.