The outer two cabin windows are designed to contain this difference in pressure between the cabin and the sky. Both the middle and the outer panes are strong enough to withstand the difference on their own, but under normal circumstances it's the outer pane that bears this pressure?thanks to the breather hole.

Every passenger airplane window is actually a sophisticated three-layer system designed to handle the extreme physics of high-altitude flight. The Outer Pane is the thickest and most critical; it is the structural layer that bears the full force of the massive pressure difference between the pressurized cabin and the thin air outside at 35,000 feet. The Middle Pane acts as a fail-safe; in the rare event that the outer pane fails, the middle pane is strong enough to maintain the cabin's pressure. This middle layer also features the famous "bleed hole"—a tiny opening that allows pressure to equalize between the cabin and the air gap between the panes, which prevents the window from fogging up or frosting over. Finally, the Inner Pane (the "scratch pane") is a thin layer of plastic that you can actually touch; it exists purely to protect the structural middle and outer panes from being scratched or damaged by passengers' fingers, jewelry, or luggage. This "triple-decker" design ensures that you have a crystal-clear, safe view of the world from the safety of a climate-controlled cabin.