Description of the scattering vector and the preferred orientation vector in the sample frame. The scattering plane appears in gray in the figure. The preferred orientation vector is defined by the azimuthal and polar angles and , respectively, in the frame. In asymmetric coplanar diffraction conditions, the scattering vector is not parallel to . In glancing incidence, the scattering vector does not possess a component along . In grazing incidence, this component is non-null. The angle between and is denoted by in this frame.