Horseshoes have long been considered lucky and should be placed over the entry door.  They were originally made of iron, a material which was believed to ward off evil spirits.  They were held in place with seven nails.  Seven being the luckiest number.  Opinion is divided as to which way the horseshoe ought to be nailed.  Some say the ends should point up so that the horseshoe catches the luck.  Others say they should point down so the luck is poured upon those entering the house.