Whether you love your window seat or prefer to stay far away and forget you’re in flight, you’ve no doubt noticed that all airplane windows are round. It’s something that you probably haven’t questioned because it’s just the way it is. But is there a reason behind it? Does the shape actually help the plane stay up, or are the curved edges just more aesthetically pleasing?

If you flew in the early days of air travel, you’ll question it even more. Plane windows were not always round—they used to be square. Check out these other 50 airplane facts you’ve always been curious about.

“The round shape, though gentle on the eyes, is for a lot more than aesthetic purposes,” says Willis Orlando, Product Operations Specialist at Scott’s Cheap Flights. “Rounded corners are designed to help distribute the pressure exerted on the window evenly, reducing the likelihood of a window cracking under changing air pressure.” These are the common myths about airplanes you need to stop believing.

And that’s not a hypothetical. It turns out when airplane windows were square, up until the 1950s, the planes flew slower and lower. As flying became more popular, airlines began to fly at higher altitudes to cut costs (there’s less drag up there, which limits unnecessary fuel use). The planes themselves also had to be increasingly pressurized, increasing the pressure difference between the inside and outside of the plane and causing more stress buildup.

That’s when square windows began to prove deadly. In 1953 and ’54, three separate planes—all de Havilland Comets, the first jet airliner—disintegrated in midair because of stress concentrates caused by the sharp edges of their windows. The analysis of the crashes, and the similar injuries on the passengers, demonstrated that “metal fatigue failure”—originating from the corners of the windows—had brought the planes down. This led to the oval design you see today—stress could flow more evenly around a round window rather than building up at the corners of a square one. With this new and improved shape, stress can flow smoothly, and we can fly safely.

So next time you’re looking out an airplane window, be very grateful that they’re that round shape! Next, find out why some airplane windows actually have little holes in them.

Sources:

Willis Orlando, Product Operations Specialist at Scott’s Cheap Flights
Aerotime: “Why Airplane Windows Are Round?”
Real Engineering on YouTube: “Why are plane windows round?”

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