A two-dimensional, graphical representation of the relationship between shear stress, confining pressure, and the angle of failure at the point at which a failure in a material occurs. Circles whose centres lie along the axis of normal stress (principal stress axes (σ‎₁ + σ‎₃/2)) and whose diameters represent the differential stress (principal stress axes σ‎₁ − σ‎₃) represent the stress state, the data for them having been obtained experimentally. Tangents to each circle join the points of failure for different stress states and so delimit the field of stable stress states (within a Mohr stress envelope) from the failure field outside.