A general position (x, y, z) is first converted by a 180° rotation about the 2-fold axis at x = 0, y = ¼ to (-x, y, ½ - z). The new z-coordinate is equal to z + (¼ - z) + (¼ - z). Then it is converted to (-x, ½ + y, ½ - z) by the translation of ½b due to the translation part of the 2-fold screw.