The round shape of airplane windows is a result of a combination of factors, including safety, aerodynamics, and historical reasons. Here are some of the main reasons why airplane windows are typically round:

1. Pressurization and Structural Integrity: One of the primary reasons for the round shape is the need for the window to maintain structural integrity under the high-pressure conditions inside the aircraft. A round shape is more resistant to stress and can distribute pressure more evenly than other shapes, which is crucial for withstanding the cabin pressure differential between the inside and outside of the plane.
2. Aerodynamicsm: Streamlined shapes are essential for reducing drag on an aircraft. The round window profile helps to minimize air resistance, which is important for fuel efficiency and overall performance of the aircraft.
3. Historical Development: Early aviators and engineers found that round windows provided the best combination of strength and visibility for the crew and passengers. As aircraft design evolved, this feature was carried forward due to its effectiveness and familiarity.
4. Safety: In the event of an emergency, such as a decompression, a round window is less likely to cause a tear or puncture in the fuselage compared to a square or rectangular window, which could create a dangerous situation due to the sudden loss of pressure.
5. Optics: The round shape provides the best optical quality for the viewing angle. It minimizes distortion and provides a clearer view of the outside environment, which is important for both passengers and pilots.
6. Manufacturing and Cost: Round windows are relatively simple to manufacture using circular glass or plastic blanks, which can be more cost-effective than complex shapes that require more precise and expensive manufacturing processes.

While round windows are the standard for most commercial airliners, there are exceptions based on specific design requirements. For example, some specialized aircraft, such as military planes or certain private jets, may have windows with different shapes to accommodate specific needs like additional armor or specialized equipment.