Basilica of St Peter
The emperor Constantine encased the tomb of St Peter in a large marble box which stood tall at the centre of the chord of the apse. The altar was a wooden table placed near the encased memorial of the saint.
Gregory the Great (590-604) decided to make the constantinian encasement into the base of the altar itself, so he added a mensa or top slab to the encasement. Because the monument was so tall, the floor level of the apse had to be raised significantly. The presidential chair was placed higher still at the head of the apse so that the Pope could see and be seen over the altar. Along with the chair there were low benches along the curved wall of the apse for the seating of the clergy.
Constantinian Basilica of St Peter
This video shows elements of the interior of the Basilica of St Peter at the Vatican in the 10th and 11th centuries. Note that much of the ambos for proclaiming the scripture. The value of this video is the reconstruction of the apse area by Gregory the Great. There are different hypotheses regarding the specifics, but this video can help a person visualize the basic arrangement.
Basilica of St Peter
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