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It is predicted by a dynamic DuMond diagram, drawn in the asymmetric Bragg case using the expressions derived by Rustichelli [Philos. Mag. (1975). 31, 1–12] for angular deviation from the Bragg law and width of total reflection, that dispersion occurs in a double-crystal (n, −n) setting when X-rays emerge from the first crystal or are incident on the second crystal with grazing angle. Experiments using the Si 220 reflection have shown a clear splitting of the Cu Kα1 and Kα2 peaks in the double-crystal rocking curve.