What is the difference between monocoque and semi-monocoque aircraft structures?

Monocoque structures carry all loads through the outer skin, like an eggshell, requiring thick skin that adds weight. Semi-monocoque structures use a combination of skin, longitudinal stringers, and transverse frames/ribs to share loads, allowing thinner, lighter skin while maintaining strength. Modern aircraft use semi-monocoque construction because it is lighter, easier to repair, and can accommodate cutouts for doors and windows while maintaining structural integrity.