[Coordinate system, see:] Euclidean (orthonormal) system . The scattering plane, defined by the direction of the incident and final wave vectors and , contains lying perpendicular and in the sense of and parallel to the scattering vector . [Polarisation of the Photon beam:] and refer to the polarisation of the beam parallel and perpendicular to , respectively. [Angles for azimuth :] , , where and is the projection of onto the plane perpendicular to . In the chosen experimental geometry azimuth , when points to the x-ray source.