The mosque, which is one of the oldest in Iran, was built in the architectural style of “four aivans”, that is, it has four gates that are opposite each other. “Ivan” is a vaulted open hall. Muqarnases – honeycomb vaults – were built on the south side of the mosque in the 13th century. During the reign of the Seljuk dynasty, two brick domed sections were completed. The southern dome was built in 1086-1087 to house the mihrab and was larger than any other mosque dome in the world built up to that time. The north dome was completed a year later; its purpose is unknown. Although it was located along a north-south axis, it is outside the mosque.