Most people have noticed that airplane windows corners are rounded. They were not always rounded. Airplanes used to have square windows, but many planes crashed because of them.
During the 1950s when planes became faster, some of them began to crash unexpectedly. Investigators found the squared-off corners of windows were susceptible to stress. Circular window corners are able to disperse that pressure more evenly.
Airplane windows now have three panes: one bears the burden of pressurization, another inner pane acts as a fail safe in case the outer pane fails, one pane faces the occupant.