The geometric shape that most commonly defined the plan of a Byzantine baptistery was the octagon.
The octagon occupied an important symbolic position in Christian architecture. Baptism was understood as a passage into new life, and the number eight was associated with resurrection and the “eighth day,” the eternal day beyond the seven-day week. An octagonal building therefore provided both a practical and theological form for the baptismal rite.
Many late antique and Byzantine baptisteries placed a baptismal font or pool at the center of the building. The surrounding walls could carry marble revetment, columns, inscriptions, and mosaics showing biblical scenes connected with water, salvation, and initiation.
The pattern was not absolute. Baptisteries could also be circular, square, or irregular, depending on local traditions and available buildings. Nevertheless, the octagon is the characteristic shape most often associated with monumental baptisteries in the Byzantine architectural world. Ravenna’s Neonian and Arian Baptisteries are famous examples.