If you’re a fan of the window seat, you’ve probably noticed a small hole at the bottom of airplane windows — and wondered why on earth that tiny thing is there.

As it turns out, the hole is called a breather hole or a bleed hole, and it serves an important safety function, according to a Slate column by pilot Mark Vanhoenacker.

The breather hole is located in the middle pane of the cabin window, between an outer pane and an inner pane, and it’s meant regulate how much pressure is exerted onto the windows. (In general, once an plane is in the air, the air pressure inside the cabin — thanks to the plane’s pressurization system — is much greater than the air pressure outside, allowing you to breathe safety.) The breather hole makes sure the outer pane bears the air pressure, so in the rare event one of the panes gives out, it’s the outer pane that goes.

There’s also an aesthetic reason for the breather hole, too — it releases moisture from the gap between the inner and outer panes, allowing the window to remain mostly fog-free.

Read next: Why No One Agrees Whether Cockpit Doors Are Safer Locked or Open

[Slate]

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