The tiny hole in an airliner window
Near the bottom of the window, a pinprick hole in the middle layer of the plastic.
Category: Getting about
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Feature. It shares the pressure load and clears fog.
Cabin windows are layered. The small breather or bleed hole in the middle pane lets the air gap equalise with the cabin, so the outer pane carries the full pressure difference. It also lets moisture escape, so frost forms on the outside rather than between the panes.
Source: The hole in your aircraft window, Migflug.