The most significant alteration carried out in the early twelfth century was the conversion of the mosque plan into a four-iwan plan 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 which 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, a massive building measuring 27 metres (89 feet) square with a huge double dome resting on squinches and muqarnas pendentives.