I've read architectural historians who describe the shape of this kind of Gothic window as a "spherical triangle". It is a three-sided window with each side a part of a circle. The first example of the design that I recall seeing is in the south aisle of Gaddesby church in Leicestershire. This dates from the period c.1325-1350. Interestingly, the tracery of that particular window includes four "spherical quadrangles" i.e. squares, but with four curved sides all of the same circumference. The spherical triangle shown above can be seen at Tewkesbury Abbey, Gloucestershire. The subject depicted in the glass is a favourite of Victorian artists, the Three Graces" (Faith, Hope and Charity). Incidentally, I can understand the need for a word or phrase to describe the shape of this kind of window, but whether the chosen one makes sense is open to argument.
photo © T. Boughen Camera: Nikon Z 5
