In some older single pane windows the grills (or grids or mullions) in the glass may actually go all the way through the glass creating a lot of individual pieces of glass in between the bars. Every square you see is a separate piece of glass. Cleaning the glass is challenging, you have to clean inside all the squares. Years ago, dual panes were made the same way. If you had a
French door with “true divided lite”, you might have as many as 30 separate dual pieces of glass in the door. Having 30 separate pieces of glass, failing at different rates was very inconvenient and costly. This is a thing of the past now.