When aircraft evolved from trusses as primary load bearing structure to sheet materials, the issue of stiffening the sheet material arose.

There are two main options:

External Stiffening: attaching some sort of stiffening to the sheet material to add sufficient strength and stiffness. Examples are stringers, stiffeners and longerons and they come in many different shapes and sizes.

• Its advantages are: easy to manufacture, cheap to join
• Its disadvantages are: inspectability, high number of parts, intersections with other structural elements

Monocoque structures: a structure made out of one piece either a panel machined out of one piece or a sandwich panel.

• Its advantages are: no stringers needed, smooth structure, high bending stiffness
• Its disadvantages are: difficult to repair, complex and expensive to join, corrosion (metal),  delamination (composites)

[photos by Saunders-Smits, 2018]