Hence, taking axes of X and F which are obtained from those of x and y by a rotation through in the sense from x towards y, we see that the particle which was at (X, Y) is moved by the pure shear followed by
the rotation to the point (X2, Y2), where
, .
Thus every plane of the material which is parallel to the plane of (X, z) slides
along itself in the direction of the axis of X through a distance proportional
to the distance of the plane from the plane of (X, z). The kind of strain just
described is called a "simple shear," the angle α is the "angle of the shear,"
and 2 tan α is the "amount of the shear."