The plane starts with a square window. But in 1953, two plane crashes occurred, and the square of the window was considered the reason for both. Where there are corners, there are weaknesses. Therefore, the square windows of the four corners have four weak points, making them easy to crack under pressure. In fact, the difference in air pressure from the inside to the outside of the window is sufficient pressure. With this in mind, the engineers decided to bypass the edges of the window to evenly distribute pressure and pressure.