Schematic description of the sample and the hkl frames. For each hkl reflection, is parallel to the scattering vector . , normal to the scattering plane, remains parallel to . lies on the scattering plane in a general position. In Bragg–Brentano geometry, is null for all reflections and the two frames are identical. As clearly displayed in Fig. 2, the rotation allows the transformation of the sample frame into the hkl frame.