This movie seeks to illustrate the three-dimensional nature of stress and to relate stresses in the $x$, $y$, and $z$ directions to Mohr's circle for stress transformations.

Problems involving stress transformations often begin with a stress element sketch, such as the one shown at the left. The stress element is commonly used to describe the state of stress that exists at a point in a deformable solid body.

It is helpful, however, to keep in mind that the square stress element is only a two-dimensional representation of a three-dimensional stress state.