All structures form in response to forces acting on rocks - and these give rise to stresses. In almost all geological situations stresses are always compressive but vary in different directions. We can evaluate the stress state in terms of the orientation and magnitudes of the so-called three principal stresses - which each act at 90 degrees to each other (i.e. they are orthogonal). Conventionally these are denoted using the greek letter "sigma". Patterns of conjugate faults - provided they formed together - can be related to the orientations of the principal stress axes. To find out how, click here.