The law of Mallard was introduced by Georges Friedel (Leçons de Cristallographie 1926, page 436) to explain, on reticular basis, twinning by pseudomerohedry.
The law of Mallard states that twin elements are always rational (i.e. direct lattice elements): therefore, a twin plane is a lattice plane, and a twin axis is a lattice row. These twin elements are pseudosymmetry elements for the lattice of the individual. The twin operations produce now slightly different orientations of the lattice of the individual, which are only quasi-equivalent, and no longer equivalent, as in the case of twinning by merohedry.