Jenna Germany historical window

Visual aid: an old European window of bullseye-derived glass with one corner pane replaced by modern undistorted glass, illustrating the historical transition.

So up to the 1600s, they were using bullseye glass. From the larger pieces you could cut out a piece of glass and get a non-bullseye piece of glass that wasn't taxed. Here's a window from Jena — Jena's in Germany or Austria, I can't remember — and up in the corner you have a nice piece of modern glass that has no distortion. The rest is the type of glass that would come from bullseye glass in ancient days. Someone broke the glass in this corner and replaced it with modern glass that's nice and flat.