a pendentive is "any of several spandrels, in the form of spherical triangles, forming a transition between the circular plan of a dome and the polygonal plan of the supporting masonry." - dictionary.com
this unique feature of architecture is a further development of churches, expanding on the early idea of the basilica. during this time period, there was an idea of religion as an institution, where people would go to churches and visually read what the architecture provided. this created a visual language, that is demonstrated in early and present day churches.
the pendentive is used in many churches, including the Hagia Sophia, which was the first masterpiece of Byzantine architecture. the dome sits at the center of the building, and is 102 feet in diameter and 182 feet in height.
another religious structure that uses the pendentive is the Sultan Ahmed Mosque, located in istanbul, turkey. it is more commonly known as the Blue Mosque, due to the blue tiles that cover its interior walls. the design of this mosque draws from a combination of Byzantine architecture and Ottoman architecture. the interior is lined with over 20,000 handmade tiles in more than 50 different designs.