 A signature feature of Parthian architecture was the iwan, an audience hall supported by arches or barrel vaults and open on one side. Use of the barrel vault replaced the Hellenic use of columns to support roofs. Although the iwan was known during the Achaemenid period and earlier in smaller and subterranean structures, it was the Parthians who first built them on a monumental scale. The earliest Parthian iwans are found at Seleucia, built in the early 1st century AD. Monumental iwans are also commonly found in the ancient temples of Hatra and perhaps modeled on the Parthian style. The largest Parthian iwans at that site have a span of 15 m (50 ft).
