Indicates the method used to obtain monochromatic radiation. _diffrn_radiation.monochromator describes the primary beam monochromator (pre-specimen monochromation). _diffrn_radiation.pdbx_analyzer specifies the post-diffraction analyser (post-specimen) monochromation. Note that monochromators may have either 'parallel' or 'antiparallel' orientation. It is assumed that the geometry is parallel unless specified otherwise. In a parallel geometry, the position of the monochromator allows the incident beam and the final post-specimen and post-monochromator beam to be as close to parallel as possible. In a parallel geometry, the diffracting planes in the specimen and monochromator will be parallel when 2*theta(monochromator) is equal to 2*theta (specimen). For further discussion see R. Jenkins and R. Snyder, Introduction to X-ray Powder Diffraction, Wiley (1996), pp. 164-5.