when stress is applied to a plane, it can be expressed in terms of one normal stress (the stress acting perpendicular to the plane), and two shear stresses (the stress acting parallel to the plane).

The simplification of the problem is required. The plane model works well when only considering a singular plane. This can be taken one step further to assume that the stress at a single point needs to be calculated, as the stress throughout a volume can vary. To calculate the stress at a point we take the simplification, that a point is in fact an infinitely small cube, and this is treated elsewhere in this program.

If you treat a point as an infintiely small cube, it is obvious that a cube has six faces, or three pairs of planes as faces. It is only important to consider three faces, as the other three parallel faces are identical in natures. These nine stress vectors are usually expressed in a stress matrix such as in seen below, and is known as the Stress Tensor. (see this item)