Under uniaxial compression, a planar fracture occurs in a cylindrical specimen of rock, while the extension of a preexisting crack is deflected toward the compression axis, creating open kinks. Our numerical models show that the maximum strain energy release rate criterion (Gmax) provides a physical basis for these seemingly contradictory observations. On a natural fracture where the friction coefficient is the same on the crack face and on its extension, the strain energy reduction is maximum if the crack extends in its own plane. This maximum is even more pronounced if the residual strain energy after the crack initiation is taken into account. In the case of an artificial crack the friction coefficient is normally smaller on the artificial cut than it would be on its natural extension; sliding is thus relatively difficult, and the maximum strain energy reduction occurs when the extension is deflected toward the compressional axis and opens. ¿ American Geophysical Union 1994