The stress distribution close to the tip of a crack which has a finite tip radius and which is being opened either by means of a remotely applied tension field σy, 0 or by means of a concentrated force (e. g. a wedge driven into the crack) has been computed. It is shown that there exist tensile stresses (σx) parallel to the plane of the crack and ahead of the crack tip. The maximum value of the tension σx is an approximately constant fraction (~ 1/5) of the peak stress concentration σy, max. which usually causes crack propagation. Inside a brittle solid, if a plane of weakness or potential cleavage is present and is roughly normal to the plane of the original crack, this interface may break and produce a secondary crack in such a manner as to interfere with the progress of the primary crack. If the ratio of the adhesive strength of the interface to the general cohesive strength of the solid is in the right range large increases in the strength and toughness of otherwise brittle solids may result.