Airline passengers may have noticed that all plane windows have rounded edges, instead of the hard corners commonly found in our house. The round windows are indeed pleasant to the eye, but they actually were designed for reasons 16 aesthetics. In the early days of aviation, plane windows 17 square in shape. Then as commercial air travel became popular in the 1950s and airplanes began flying higher and faster, three planes mysteriously broke apart in midair. The cause? Square windows. Scientists found that sharp corners are natural weak spots where stress concentrates. The problem is 18 when airplanes fly at higher altitudes, where the difference between the inside and outside pressure increases, causing added stress. When subjected to repeated pressurization high in the sky, the four corners on a square window may 19 . Curved windows, on the other hand, distribute stress around more evenly, reducing the likelihood of cracks or breaks. Circular shapes are also stronger and resist deformation, and therefore can 20 extreme differences in pressure inside and outside of an aircraft. Thus, round windows are a major safety innovation that keeps planes from disintegrating mid-flight. They are also used on ships and spacecraft for their greater structural integrity.