Pendentives are the triangular curving segments that support a dome and transfer its weight to the pillars below. They were introduced by Byzantine architects who first used it on a large scale at Hagia Sophia in today's Istanbul. Its use allowed for more extensive unobstructed interior expanses as well as the enclosure of a square opening with a circular dome. Pendentives provide surfaces that can be decorated with either mosaics, frescoes, or sculpture reliefs. Examples of pendentive mosaics include the Four Doctors of the Church in the Gregorian Chapel at St. Peter's (1580s), executed by the Mannerist artist Girolamo Muziano. Pinturicchio frescoed the pendentives in the retro-choir of Santa Maria del Popolo, Rome (1508), with the same theme. Terracotta reliefs by Donatello in the pendentives of the Old Sacristy, San Lorenzo, Florence (1421-1428), and those by Luca della Robbia in the Pazzi Chapel (1433-1461) in the same city provide sculptural examples.
Historical dictionary of Renaissance art. Lilian H. Zirpolo. 2008.