Structures can be classified in many ways according to their shape, their function and the materials from which they are made.

The most obvious definition of a shell might be through its geometry. A structure or structural element may be a fully three-dimensional solid object, or it might have some dimensions notably smaller than others. A beam is straight and it is relatively long in comparison to its cross section. Thus it is defined by a straight line. An arch is defined by a curved line and a plate by a plane.