f81r parallel

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Afterthought: Although I've never seen a baptismal font big enough for three adults. They're not hot tubs, they're designed for dipping a baby's head in.

This is something I should have explained in more detail in my post.
The reason the fonts changed from sunken to pedestalled was because of the changing practice of baptism, from immersing adults (which requires a full-size bath or tub, therefore, the preference for a large, often sunken font) to the generalization of the baptism of newborns, and just dipping the baby's head, which is much more conveniently done in a raised, smaller font.
Here are some images of the sunken fonts. Because of changing ground levels, some of them now appear exposed rather than sunken:
Turkey, 5thC:

Tunisia, 6thC:


Germany, 11thC: