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 The direction of maximum flow or slip is termed 'a' axis, the plane ab is the slip plane and c is the slip plane normal. The terminology first evolved at the time the concepts for petrofabric analysis were being evolved by Bruno Sander. Later, the same became applicable to similar folds since these were the only folds in which the cleavage planes were not of maximum longitudinal strain but of maximum displacement. Then the application came to the area of plate tectonics and movement of thrust sheets. Lastly, it became applicable to sheath folds and with the impetus on research on shear zones, the terminology has now become applicable to sheared rocks. Though the concept has become widely applicable, the usage of the very terms a, b and c  appears to have been done away with.  In orogenic deformation, the movement direction of thrust sheets is  also 'a' or tectonic transport direction. Usually the thrust sheets are emplaced and are accompanied by transpression, a term coined by Harland in 1971.