Near-vertical fracturing, jointing, microcracks, and stress tends to develop a horizontal axis of symmetry perpendicular to the fracturing called azimuthal anisotropy (sometimes, extensive dilatancy anisotropy). The velocity of P-waves parallel to the fracturing and of S-waves whose motion is polarized parallel to the fracturing is larger than for those perpendicular to the fracturing. This anisotropy is usually a form of orthorhombic anisotropy with the fracturing, and it involves nine independent elastic constants. It is sometimes considered a type of polar anisotropy (q.v.), and the less important elastic constants are often assumed, sometimes neglected, or effectively set to zero. The symmetry axis because of the fracturing may not be orthogonal to the axis because of the layering, involving monoclinic anisotropy. This situation is involved in birefringence or shear-wave splitting (q.v.).