View source for Mallard's law

<font color="blue">Loi de Mallard</font> (''Fr'').  <font color="red">Mallard-Gesetz</font> (''Ge''). <font color="black">Legge di Mallard</font> (''It''). <font color="purple">マラード法則</font> (''Ja''). <font color="green">Ley de Mallard</font> (''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 element]]s are always rational (''i.e.'' [[direct lattice]] elements); therefore, a [[twin element|twin plane]] is a lattice plane, and a [[twin element|twin axis]] is a lattice row. These twin elements are pseudo-[[symmetry element]]s 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]].

[[Category:Twinning]]