Torii are built at the boundary of the inner sanctum and the outside of shrine. The origin of the archway is thought to lie in Japanese mythology with the fallen log on which a bird (tori) was perched to draw the Sun Goddess Amaterasu out from the heavenly rock cave she had hidden in.
Originally torii were made from wood but later torii began to be made from stone and steel. Wooden torii were cut into square and then octagons to remove the rough bark and then gradually cut to form a circle which was finished with a circular plane.
There are jinmei torii with a straight top, myojin torii with curved ends, sanno torii which have a triangular attachment and inari torii which are painted red.
出典: 日本伝統文化の英語表現辞典