I could get very detailed about this, describing the anatomy of a horses foot, but I'm not going to. The foot has sole, and a hoof wall. Between the sole and wall, there is a thin white line. The hoof is trimmed and rasped level. The shoe is shaped, and the nail holes angled (with a pritchel) so the nail can enter the foot right on the white line, at the same angle of the of the hoof wall. The nail is a soft iron, with a tip designed to cause the nail to come out of the hoof, which is why you see the horseshoer taping the nail. He goes tap, tap, tap, whack, and the nail pops out right where he wants it to. The hard whack bend the nail to the outside. He then cuts the nail of with a twist of his hammer, and finally clinches the nails tight to hold the shoe on. Usually four nails are used to a side of each shoe, however three nails to a side will hold the shoe on a sound, healthy foot. You try to drive a nail inside the white line, and you are into the quick, and the horse will be lame. Outside the white line, the hoof will split at the worst, and at the best the nail won't hold. So I expect that there was never a time in history that double rows of nails were used. For unhealthy or broken up feet the shoe is worked red hot, and toe clips or side clips are drawn out, then burned into the foot. Perhaps only one nail will hold on a broken away hoof wall on one side, and a couple on the other, then the clips burned in will hold the shoe long enough for the foot to grow out and the horse is still useable in the mean time.
