The imaginary line drawn through the centre of the three phalanges. From the front (dorsal) or back (palmar/plantar) aspect, the line should equally bisect the hoof and pastern and be perpendicular to the ground. From the side, the line should be straight and parallel to the line of the dorsal hoof wall, if this axis is not straight, one differentiates between a broken forward phalangeal axis or a broken back phalangeal axis (or HPA).