Analytical work on the aspects of the stress field that cause a slant fracture to start forming is described. The "build-up" distance for transverse stress and the orientation of the stress field both point to the importance of the quantity (in- - 1)R for a circular contour. It is shown, further that a unit zero isoclinic beginning from the location defined by this quantity is practically an osculating curve of the unit circle. Using this interrelationship, it is demonstrated that fractures, in a number of sections from crack-notch toughness specimens, follow the elastic zero isoclinic precisely even though considerable plastic flow Preceded the fracture. For cup-cone fractures in round tensile-test specimens, this same quantity appears to define the interrelationship between surface contour and the width of the cone.