In most purchased (rather than home made) sets of runes, there is a blank rune. Some people claim this rune has a meaning of it's own, while others believe it is included as an extra in case one of the runes is lost or ruined. Often times, those books on spells or divination that include the blank rune as a rune in it's own right do not include the history to back it up, while a quick google search shows claims that it was invented by some dude in the 1970's.

For those who practice or study rune magick, what side of the debate are you on?