From Online Dictionary of Crystallography
Spécialisation métrique (Fr). Spezielle Metrik (Ge). Specializzazione metrica (It). Especialización métrica (Sp).
The term metric specialization indicates a special relation between cell parameters which bypasses the restrictions imposed by the symmetry of the crystal structure. For example, the symmetry of an orthorhombic crystal structure imposes that the interaxial angles of the conventional cell are all right angles. It does not however impose any restriction on the linear parameters, so that a, b and c can take any value. As a special case, it may happen that two of them (possibly all of them) are equal within the standard uncertainty. Because this equality is not imposed by the symmetry of the crystal structure and because the thermal expansion is in general different along the three axes, the metric specialization is realized only within a certain interval of temperature and pressure. By changing the experimental condition, one should be able to observe a departure from this specialized metric, without any phase transition taking place. If a single experiment is performed, the metric specialization may suggest a higher symmetry than real.
The occurrence of metric specialization is at the origin of twinning by metric merohedry.