Accounts of medieval windows generally focus on stained glass, and no wonder—they're very beautiful. But, of course, not all windows were tinted. Recently, I came across a complicated allegorical frontispiece on fol.1r of a French Mirror of History. Half the picture depicts a church being built with various kings, saints, and biblical figures as craftsmen—including these two monks. They grabbed my attention because I had never seen a depiction of glaziers installing windows.
I collect pictures of medieval life. Among my categories are domestic interiors and craftsmen at work. Rooms in 15th C manuscripts often have windows with plain glass, and this image suggests that diamond panes were set in sections which were in turn set into the window frame. Once you understand how it was done, moreover, you can see evidence of the practice in illuminations of interiors. Notice the shutters, frames, and leading shown in the details of the windows in a court scene in a French chronicle history (you can zoom the image).
Writers of historical fiction need to be magpies, picking up information wherever they find it and storing it away. And what a little ping of pleasure an odd sighting can give!