"Section of the Church of S. Constantia at Rome. The simplest desctiptions of this kind of building are the baptistries, for which the ancient Thermæ furnished models. They generally have a circular or octagonal ground plan, a main space covered with a round or polygonal dome, and a circular passage separated therefrom by pillars in the same way that the side aisles are separated from the main aisle in basilicas. Sometimes they were without this passage, and only had galleries running round the interior like boxes in a theater. These buildings were generally constructed in the vicinity of cathedrals.