When the barrier was low, it could be of wood generally, except in Egypt, only the imprints remain or metal or stone. The most common type was made of newels with various terminations spheres, balls, cones, cubes etc. and with a system of panels, decorated or not, slotted in. The same type served as a balustrade in the tribunes. The decoration of the marble slabs, mass-produced in the eastern quarries Proconnesus, Thasos etc., was stereotyped in the 5th and 6th c., and numerous examples remain in loco Constantinople, St. Mark’s in Venice or in museums.
More decorative models are known, e.g., esp. the openwork panels preserved in the Museo Nazionale di Ravenna. The barriers were interrupted naturally by entrances closed by doors, generally of metal, or by chains. There were upright cancelli in the apse, esp. when this was raised, and around the space reserved for the altar. The most common type was a quadrangular enclosure the width of the apse and set in front of it, with an axial opening generally to W and two side openings permitting passage across.
But this enclosure could be separate from the apse and very far away when the altar was in the center of the nave. In this case a corridor, protected by two barriers, would provide a link. This type of enclosure is usually called a choir, from the name given to the group of cantors chorus who took part in the liturgy which is ambiguous since the choir sometimes occupied an enclosure distinct from that of the altar e.g., in Spain or sometimes a sanctuary a name also given by some to the apse. A corridor, generally called by Italian specialists a solea from the name given at Hagia Sophia, Constantinople, to the bridge joining the enclosure wall to the ambo sometimes led off from the altar enclosure. In effect, there was sometimes a corridor which ensured the link between the choir and an ambo in the axis.
But a short corridor might be intended only to protect the axial entrance to the choir, while a long corridor could ensure the link either with the main door for processions a case not archaeologically proven or with a second center of worship see below: double apses. Finally, there are cases of double enclosures, the second one of equal length examples in Lebanon or slightly less wide than the altar enclosure. This secondary type is well attested in Roman churches and is usually called schola cantorum, since the clerics responsible for the sung part of the liturgy were placed here from the early Middle Ages on. The organization of the church could be much more complex still. It often happened that the altar was protected either inside the choir by a second enclosure between the columns of the ciborium or outside the choir; other enclosures delimited sectors perhaps reserved for privileged categories an African inscription speaks of cancellus virginum: for consecrated virgins?; in Africa, Greece and Spain, we also see cancelli completely cutting off the aisles from the nave, which seems reserved for the movements of the clergy.