The chances are that your bedroom window is either square or rectangular. Your airplane window, however, will always be circular.
While this is an obvious statement, the fact that your house is designed to stay on the ground while an aircraft is designed to fly in the air explains the difference in window designs. Unlike your house, an aircraft is subject to differing levels of atmospheric pressure, which must be considered when designing aircraft windows.
The first aircraft was designed with square aircraft windows. It wasn’t until the 1950s, with the advent of commercial jets, that all aircraft were fitted with circular windows. As commercial jets flew higher in altitude, the difference between the pressure inside the cabin and outside the cabin became more significant and apparent.
As an aircraft climbs higher in altitude, the chance in atmospheric pressure causes the fuselage to expand slightly. This results in the materials of the aircraft warping or shifting shape. The repeated change in the materials causes strain that, if not properly monitored and accounted for, can lead to permanent damage.
Atmospheric pressure is the reason for the cylindrical shape of an aircraft. Pressure can easily flow through the body of a cylindrical aircraft. Any deviation or obstruction, such as the corners of a rectangular window, interrupts the flow of pressure within an aircraft, making it such that the pressure can no longer flow easily. On the other hand, circular windows have minimal interruptions.
Imagine this. If you placed a ball within a bowl it is able to roll around the sides uninterrupted. If you then decided to add ridges to the inside of the bowl, the ball would get stuck behind the ridges and the rolling would be interrupted. The ball would be able to move past the ridge but, over time, the ridge be worn down. In an aircraft with rectangular windows, the flow of pressure is caught in the corners of the windows. The buildup of pressure on the windows may not be noticeable at first, but over time, the windows weaken. If all the windows on an aircraft are weakened, the overall integrity of the aircraft is called into question, therefore making it unsafe to fly.
Circular windows do not have a focal point in which pressure can collect. The pressure flow will be interrupted by the introduction of windows however the circular window design minimizes the disruption. The pressure skims over the softer edge of the window rather than jauntily negotiating the corners.
The windows themselves have three layers: outer pane, the middle pane and the inner panes. The purpose of the multiple panes relates back to the topic of pressure. The outer pane is the actual window where the pressure is applied. At 35,000 feet the atmospheric pressure is 3.4 pounds per square inch. Cabin air pressure is maintained at 11 pounds per square inch. The bigger the pressure, the more robust the window panes must be. Therefore, aircraft windows are made of acrylic material that is both resilient and, importantly, for a window, transparent.