Reading this, I can intuitively understand that fractures propagate along the path of least resistance, creating "width in a direction that requires the least force". However, it is less intuitive (to me, at least) that "a fracture will propagate parallel to the greatest principal stress and perpendicular to the plane of the least principle stress". In my mind, I visualize a rock between the two plates of a press. I can see the forces balancing until a critical force is applied. At this point, how do I visualize the stresses? Do I have to analyze the stresses at a molecular level or is macroscopic description sufficient?