Seljuk architecture
The most significant change was in the early 12th century. It was the conversion of the hypostyle plan into a four-iwan plan mosque (see Shah Mosque). Another mosque-type introduced at this time was the kiosk mosque, consisting of a domed space with three open sides and wall containing a mihrab on the qibla side. The architecture of this period was also characterized by memorial tombs. These were usually octagonal structures with domed roofs, called Kümbet or Türbe. An impressive example of tomb architecture is the mausoleum of Sultan Sanjar at Merv. It is a massive building measuring  square with a huge double dome resting on squinches and muqarnas pendentives.