Until the 6th century the altar of Christian churches would have been in full view of the congregation, separated only by a low altar rail around it. Large churches had a ciborium, or canopy on four columns, over the altar, from which hung altar curtains which were closed at certain points in the liturgy. Then however, following the example of the church of Hagia Sophia in Constantinople, churches began to surround their altars with a colonnade or templon which supported a decorated architrave beam along which a curtain could be drawn to veil the altar at specific points in the consecration of the Eucharist; and this altar screen, with widely spaced columns, subsequently became standard in the major churches of Rome. In Rome the ritual choir tended to be located west of the altar screen, and this choir area was also surrounded by cancelli, or low chancel screens. These arrangements still survive in the Roman basilicas of San Clemente and Santa Maria in Cosmedin, as well as St Mark's Basilica in Venice. In the Eastern Church, the templon and its associated curtains and decorations evolved into the modern iconostasis. In the Western Church, the cancelli screens of the ritual choir developed into the choir stalls and pulpitum screen of major cathedral and monastic churches; but the colonnaded altar screen was superseded from the 10th century onwards, when the practice developed of raising a canopy or baldacchino, carrying veiling curtains, over the altar itself.