Amy Knight Powell at Cabinet Magazine:
Scholars have built a tight historical and corresponding conceptual link between the advent of the rectangular, Renaissance frame and the advent of linear perspective. It’s true that the renovation of Fra Angelico’s San Domenico altarpiece entailed both, but this was usually not the case. In 1480, about a century and a half after it was made, Giotto’s altarpiece for the Baroncelli Chapel in Santa Croce was renovated. Its original frame was removed and discarded, along with God the Father, who had occupied the gable above the heads of the Virgin and Christ. Spandrels decorated with vermilion cherubim were added, along with a new, rectangular frame that slices right through the curves of the cusped arches.
No new background in perspective was needed. To bring the painting up to date, it sufficed to change its shape—into a rectangle. This shape became so prevalent in Italy by the end of the fifteenth century that paintings came to be called quadri, even when they were round.