Form of the baptistery. When the baptistery was a room attached to the church, its form could be neutral a simple rectangle. Sometimes an apse was added, which could contain the bishop’s chair or an altar see below. The ideal form of the separate baptistery e.g., the Lateran, sometimes also of that attached to a church, was a central plan, inherited from the common forms of mausoleums, baths or palace vestibules: square, circular, polygonal, polygon inscribed in a square with corner niches, tetraconch, triconch, etc., with the pool occupying the center. The central part of the room could be raised; i.e., it could have a row of internal columns supporting a lantern or the drum of a dome and separating the pool from a peripheral ambulatory as in the baptisteries of Provence.
At a later date Christian symbolism was applied to earlier forms: particularly often cited are Ambrose’s verses for the octagonal baptistery of Milan, which seems to have been imitated, esp. in N Italy, from the end of the 4th c. For the geographical distribution of these different forms, see Khatchatrian’s table of inventories. 4. Form of the font. The baptismal pool was a basin derived directly from those of the thermal bath or the pools of peristyles. Their form, dimensions and depth varied according to time, the dimensions of the room and the baptismal rite. In general we observe an evolution from simple forms circular or square to complex symbolic forms cruciform and quadrilobate, passing through polygonal forms esp. hexagonal and octagonal. The polygonal form is well attested from the end of the 4th c. in N Italy in baptisteries which are also polygonal. Cruciform shapes were used esp. in the Byzantine era 6th c.. Almost exclusive to N Africa is a series with more than four lobes 6, 8 or 12, also of Byzantine date, which should perhaps be compared with polylobate altar tables on account of the link between the altar and the baptismal font, both instruments of redemption.