Early passenger jets didn't always have the rounded windows we recognize today.

In the 1950s, the de Havilland Comet suffered catastrophic in-flight breakups. Investigators found that repeated pressurization of the aircraft cabin caused metal fatigue, with dangerous stresses concentrating around openings in the fuselage.

Sharp corners make those stress concentrations worse. Rounded windows distribute the stresses more smoothly and eliminate the vulnerable corners.

Modern airliners incorporate that lesson throughout their pressurized structures.

So those rounded windows aren't an aesthetic choice.

They're part of what keeps the airplane in one piece.