a Laue class
/ LOW-uh /
When you shine X-rays through a crystal, the diffraction pattern always comes out looking centrosymmetric, even if the crystal itself is not. Friedel's law says the reflection (hkl) and its opposite (-h,-k,-l) have essentially equal intensity, so diffraction hands you a symmetry that includes an inversion centre you may not really have. A Laue class is the symmetry group that diffraction actually reveals: the crystal's true point group with a centre of inversion forcibly added.
Because adding a centre collapses distinctions, the 32 crystal classes fold down into just 11 Laue classes, exactly the 11 centrosymmetric point groups (1-bar, 2/m, mmm, 4/m, 4/mmm, 3-bar, 3-bar-m, 6/m, 6/mmm, m-3, m-3m). Every one of the 32 classes maps to whichever of these 11 you get by throwing in an inversion centre. So several different crystals share a Laue class: point groups 4, 4-bar and 4/m all diffract with Laue symmetry 4/m.
The Laue class is what you can read straight off a diffraction pattern's symmetry, so it is your first, robust clue to the crystal system and point group when solving a structure. But it is only a clue: because it cannot distinguish a crystal from its inverse, it cannot by itself tell centrosymmetric from non-centrosymmetric, or a left crystal from a right one. Getting past that ambiguity, using intensity statistics, anomalous scattering, or extra physics, is part of the real work of structure determination.
Point groups 422, 4mm, 4-bar-2m and 4/mmm all show the same diffraction symmetry: Laue class 4/mmm. Diffraction alone cannot tell them apart.
A Laue class is the diffraction-visible symmetry, the point group plus a forced inversion centre; there are 11.
The 11 Laue classes exist because diffraction obeys Friedel's law and adds a centre you may not have. So a Laue class narrows down the point group but never uniquely fixes it, and cannot prove a crystal is centrosymmetric.