See Plate XVI facing Works, 10.300 for an illustration of fourth order windows, and the comment at Works, 10.299f.
For a general account of the orders of window and door arches see Door Book.Front pastedown; House Book 1.Front pastedown; and compare Works, 10.290ff and the Plate XIV facing Works, 10.290.
See Constantini & Zannier (1986) p.15, where this page is reproduced.
[Version 0.05: May 2008]