Originally, paper was used, but nowadays, glass is used instead of paper.
It consists of 3 glass textures and 3wood textures.
There are transparent, translucent, and a mixture of translucent and translucent swatches.
Windows for 2 tiles have 6 swatches (3 wood textures, 2 glass textures)
Windows for 4 tiles have 9 swatches (3 wood textures, 3 glass textures)