Metal nails, particulary those of iron or steel, are valuable offensive or defensice weapons against witches, goblins, fairies, pixies, and similar beings. A nail carried in one's pocket provides protection against being led astray by pixies. (Because the nail keeps its possesor fixed onto the right track.) A nail driven into a wich's footprint will cause her such crippling pain that her activities are greatly restricted. Nails driven into a door, especially when several are insered cruciform, prevents many different types of beings from entering the home. A nail laid on any item left in the open air will prevent the fairies from spiriting it away, because the 'fixing power' of a nail is greater than the levitational powers of fairies. If it is necessary to make a midnight journey, a few nails scattered in one's tracks will discourage pursuit by nightwalkers.
Various theories are advanced to explain the potency of metal nails. Some people say that they have represent the nails of the Crucifixion: others that supernatrual beings fear their mystic power of securing one object to another. Whatever the reason, a supply of nails provides cheap and easy protection for the average mortal.
Taken from "Encyclopedia of THINGS THAT NEVER WERE" by Michael Page and Robert Ingpen