Why do airplane windows have tiny holes in them?

Airplane windows are made of three layers of acrylic. Those tiny holes—often called "breather holes"—are located in the middle pane.

They serve two primary purposes:

1. Pressure Regulation: As a plane climbs, the air pressure inside the cabin is kept much higher than the air pressure outside. The breather hole allows pressure to equalize between the middle and outer panes. This ensures that the outer pane bears the brunt of the pressure load, keeping the cabin pressurized safely.
2. Anti-Fogging: The hole also helps manage moisture. By allowing air to circulate between the panes, it prevents condensation and frost from forming in the gap, ensuring the window stays clear so you can see outside during the flight.