(...)the area around the altar was also considered holy because of the physical presence of God in the Eucharist (communion bread, which Catholics considered to have been 'transubstantiated" into the actual body of Jesus), both during the Mass and in the tabernacle on the altar the rest of the time. So that people could tell when Jesus was there (in the tabernacle), the "sanctuary lamp" would be lit, indicating that anyone approaching the altar should genuflect (bow by bending the knee and inclining the head), to show respect for Him. In most Eastern Orthodox churches, the sanctuary is separated from the nave (where the people pray) by an iconostasis, literally a wall of icons, with three doors in it. In many Roman Catholic churches, altar rails mark the edge of the sanctuary.