If you can't see the interference, rub carpenter's chalk on the inside of the hoof to determine where the interference is occurring and if one or both feet are involved. When trimming the base narrow horse, the inside hoof wall should be lowered. This provides more clearance when the horse moves. If the horse is cow-hocked, whether or not it interferes, it should be trimmed in such a manner that the outside hoof wall is trimmed lower. This will change the position of the foot, placing the toes more in line with a straight flight path.
Farriers find that a square-toe shoe is useful, as it helps to straighten the path of flight.
They will trim and shoe accordingly to prevent interference. Forging occurs when the toe of the hind foot strikes the heel or bottom of the front foot on the same side. The front foot is too slow in leaving the ground to avoid the forward advancing hind foot. Overreaching is a more severe form of forging. As the advancing hind limb approaches the fore limb, the toe of the hind foot lands on the heel bulb of the front foot, even before that front foot leaves the ground.