We introduce a method to infer the remote differential stress magnitude from the curvature of overlapping echelon fracture traces. The curving paths of overlapping echelon cracks imply the predominance of local crack-induced stresses over remote stresses during propagation. Nearly straight crack paths imply the controlling influence of a remote compressive crack-parallel differential stress. This method is used to interpret complex joint patterns mapped in sedimentary rock. It is also applied to the problem of fracture-pattern generation using computer models.