The stress ellipsoid is useful for visualizing the orientations and relative magnitudes of the principal stresses at a point, but it is not very convenient for showing the relationship between the orientation of a plane and the magnitudes of the shear stress and normal stresses on the plane. In particular, it would be useful to have a diagram which illustrated the relationship between the principal stresses and the normal and shear stresses on some inclined plane. There is a diagram, called a Mohr diagram, that illustrates these relationships. We will find this diagram particularly useful when we discuss joints and faults, and a modification of this diagram is useful for solving many problems concerning strain.