During the Three Kingdoms period (220–280), the "cross-joint dome" ("siyuxuanjinshi") was developed under the Wu and Western Jin dynasties south of the Yangtze River, with arcs building out from the corners of a square room until they met and joined at the center. These domes were stronger, had a steeped angle, and could cover larger areas than the relatively shallow cloister vaults. Over time, they were made taller and wider. There were also corbel vaults, called "diese", although these are the weakest type. Some tombs of the Song Dynasty (960–1279) have beehive domes.