Naturally occurring extension fractures, including some joints and veins, form sets consisting of numerous subparallel fractures, each with well-defined ends. In order to correctly interpret these fractures in the field, it is necessary to determine which factors control the geometries of fracture sets. This requires consideration of the conditions leading to both the initiation and the cessation of fracture propagation. A fracture begins to propagate when the crack extension force reaches a critical value, which is a property of the rock and of environmental conditions. After propagation initiates, the crack extension force varies owing to: (1) increase in crack length, (2) change in remote strain, (3) elastic interaction with nearby cracks, (4) change in internal fluid pressure, and (5) stress relaxation due to growth of cracks in the surrounding rock. Each fracture continues to propagate as long as its crack extension force exceeds the critical value and terminates when this condition is no longer met. Neglecting fluid effects, the final geometry of the fracture set is a function of the remote strain history and the initial crack geometry, which can be characterized by the initial crack density. For low initial densities, large increases in crack length occur with little or no increase in remote strain. However, for large initial densities, crack lengths increase only if the applied strain increases. This analysis is used to estimate the variation in remote strain during formation of a fracture set in granitic rock from observations of the final fracture geometry.