Let us, to begin with, consider a spherical rock element in a static region of the Earth’s crust. The stress (force/unit area) acting on the element will be the same in all directions. Such a stress condition is termed hydrostatic and has been defined by Anderson (1951, p. 13) as the standard state. However, the stress state in the Earth’s crust is often not truly hydrostatic; nevertheless any balanced system of stresses, whether caused by compressional, tensional or torsional forces, can be resolved into three principal stresses at right angles to each other (Hills 1953 p. 23 et seq., Price 1966). These three principal stresses can be symbolised as Pmaximum, Pintermediate and Pminimum. Alternatively they are often referred to by using the Greek letters σ v σ 2 and σ3 respectively.