* a * axis [struc petrol]

One of three orthogonal reference axes, * a, b, * and * c, * that are used in two distinct ways. (a) To help describe the geometry of a fabric possessing orthorhombic or monoclinic symmetry. The unique symmetry plane is labelled the * a-c * plane, and * a * lies at the intersection of this plane with a prominent fabric surface. (b) In a kinematic sense, to describe a * deformation plan * that possesses orthorhombic or monoclinic symmetry, such as progressive pure or simple shear. In this case the * a * axis again lies in the unique plane of symmetry but parallel to the movement plane. It is the direction of maximum displacement and is commonly referred to as the direction of tectonic transport. In a progressive simple shear the * a * axis is the direction of shear. See also: * b axis [struc petrol]; c axis [struc petrol] * . Syn: * a direction * .