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.