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Airplane windows are rounded to reduce the stress from the pressurization. Engineers learned this the hard way after a series of fatal accidents in the mid-1900s involving airplanes with square windows. They conducted experiments in water tanks to simulate compression and decompression on a plane, and discovered that the airplanes came apart in midair because sharp corners, like those on the window, are ideal locations for stress concentrations. Now, all airplane windows and doors are rounded to reduce the stress concentrations.