Contrary to popular belief, viscoelastic Earth material can adjust itself to its pressure environment over extended periods of geologic time. When this happens, the internal shape of the material then possesses an internal equilibrium pressure. Solid rocks are expected to contain shear stresses that are dependent on the configuration of the rock and the difference between their current pressure environment and the equilibrium pressure. This pressure could date from the rock's formation, an episode of metamorphism, a tectonic deformation, or a long period of slow, viscous relaxation. We should regard the internal equilibrium pressure as an intrinsic physical property of Earth materials.