During the period of the Second Temple, the Heichal was one hundred cubits high, so it was not possible to build a wall that high with a thickness of only one cubit, for such a wall would have been unstable. Therefore, two curtains were hung, with a distance of one cubit between them as an alternative to the wall. When the High Priest would enter the Holy of Holies through these two curtains, he would enter on the southern side and walk between the two curtains. When he reached the northern wall, he entered the Holy of Holies. (Sources: Rambam Beit Habechira 4,2; Mishna ,Yoma 5,1; Gemara, Yoma 21) 
Uses
Contents:   Part I,    Part II,    Part III,    Part IV,    Part V,    Part VI,    Part VII