Circular and polygonal plans appear in a number of Syrian examples of the early sixth century. Their most striking feature is the inscribing of the circle of polygon in a square which forms the exterior outline, and the use of four niches to fill out the corners. This occurs at Kelat Seman, a small double church, perhaps the tomb and chapel of a martyr; in the cathedral at Bozrah, and in the small domical church of St. George at Ezra. These were probably the prototypes of many Byzantine churches like St. Sergius at Constantinople, and San Vitale at Ravenna.