A pane is a single sheet of glass in a window or door. Many windows are made up of several panes held in a frame.
From Old French 'pan' meaning 'piece' or 'section', from Latin 'panis' meaning 'piece of cloth' or 'patch'. It came to mean a separate section of a window made of one piece of glass.
A pane of glass is basically just a 'piece'—the word doesn’t originally care what material it is. Language quietly reminds us that big things, like windows and walls, are often just collections of smaller pieces.
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