The prayer hall of the Yıldırım Beyazıt mosque follows the ‘reverse T’ ground plan, with a central space (on the foreground of this picture), flanked by three iwan-rooms: the ‘mihrab’ area in the South (in the background) and two aisles (East and West; not visible on this picture).
The arch that separates the central space from the mihrab area is called ‘Bursa arch’; this mosque was the very first one in which this peculiar form of arch was used.

An iwan (Persian & Turkish: ‘eyvān’) is a rectangular hall or space, usually vaulted, walled on three sides, with one end entirely open. This architectural form can be used for entrances, but also (as it is the case here) to create half open rooms looking out at a central yard. The iwan as used in Anatolian Seljuk ‘medrese’ was imported from Islamic Persia, but was invented much earlier and fully developed in Mesopotamia.

Correspondent: J.M.Criel, Antwerpen.
Source: ‘Vakıf Abideler ve eski Eserler (volume III)’ - Vakıflar Genel Müdürlüğü, Ankara 1983 & Vikipedia.