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A study of the extent of the three-dimensional region around the border of a straight through-crack in mode-I loading was undertaken by the method of caustics. A series of experiments were performed in single edge-notched specimens made of PMMA (Plexiglas). It was found that the state of stress very close to the crack tip can be approximated as plane strain. Following this region, three-dimensional stress conditions hold in the neighborhood of the crack tip, up to a critical distance which depends on the thickness of the plate and the crack length. Beyond this critical distance plane stress conditions dominate. Results were obtained for the extent of the three-dimensional region as a function of the specimen thickness. The present investigation illuminates the complicated problem of the changing state of stress around cracks and establishes the limits of applicability of the method of caustics for the determination of stress intensity factors.