Minecraft wiki entry for Glass_Pane, associated text: If placed without anything on the sides, they appear as a thin vertical • shape, 2×2 pixels in size; but when other blocks occupy adjacent spaces, it becomes flat or turns into a or or shape depending on the items surrounding it. The collision box is identical to the shape of the pane. Therefore, two glass panes placed with one empty space in between forms a gap that is two blocks wide.