The Black Count over at Convent of Blood had a very interesting discussion the other day: why does a vampire need to be invited into a mortal's home (at least, according to certain myths -- Angel, Lost Boys, Fright Night, Let the Right One In)? What keeps him (or her!) from entering of his own free will?
Thinking about this, I wondered if it had something to do with magical association of threshholds, and Taliesin reaffirmed my thoughts in his comment to the Black Count's post. So I did a bit of research into threshholds (mind you, on the internet, so there are bound to be gaps -- next time I'll do some good old-fashioned 'crack open the books at the library' research ;-)
I remember from watching HBO's Rome (what -- good research doesn't cite TV shows?! ;-) that thresholds had their own god, Janus, and one would pray to him when one was moving from one stage of life to another (like when Vorenus left the army to open up his own business).
Quite possibly this had to do with the fact that the threshold of a home, where one lived, slept, and raised a family, was considered the gateway between safety and the big, bad, scary world outside, where cold weather, disease, and pillagers were present. Thus, an idea grew, wherein blessing or making certain signs, planting rosemary, hanging horseshoes above or near the threshold of a house would ward off the big, bad evil that lay in wait in the outside world. As vampires were a big, bad evil, the threshold philosophy applied to them, and thus the idea that they were bound by magic to stay outside the home unless invited in came about. More about this idea is presented on this webpage here.
Of course, it also makes a great plot device in books, tv, and movies, and the mysterious, magical lore surrounding vampires is what makes them so popular and fun to write about/watch.
I can remember this bit of lore used strongly in The Lost Boys and Fright Night, but how about you readers out there? Do you agree with the threshold theory, or is there something more? We'd love to hear your thoughts, as would The Black Count over at his site here!