Because of the fluid nature of glass, glass panes fastened to windows or doors in historic structures are thicker towards the bottom. The glass flows down extremely slowly, thickening the bottom section slightly. As it does so, it reaches the end of the pane and has to be cut off, leaving a thin edge. The thicker bottom section then becomes the base for further repairs over time. In conclusion, the window glass of the old buildings is thick at the bottom because glass tends to flow downward slowly, thickening the bottom section slightly. This is how historians know that these structures date back many years ago.