Mallard's law
From Online Dictionary of Crystallography
Loi de Mallard (Fr). Mallard-Gesetz (Ge). Legge di Mallard (It). マラード法則 (Ja). Ley de Mallard (Sp).
The law of Mallard was introduced by Georges Friedel [Leçons de Cristallographie (1926), p. 436] to explain, on a 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 pseudo-symmetry 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.