By Kentucky Equine Research - The white line is the narrow, light-coloured band visible on the underside of a freshly trimmed hoof at the junction of the hoof wall and the sole. White line disease, an infection that causes separation of the wall, may be seen first at the white line but actually affects the zone of contact between the hard outer hoof wall and the middle layer of hoof tissue. It occurs most commonly in front feet but can occur in any foot.