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The conditions which give high-resolution monochromatization of neutrons by multiple Bragg reflection in hexagonal close-packed crystals have been studied theoretically. The role of reflections forbidden by special atomic positions has been clarified, and, contrary to a previous conclusion, it is found that these `forbidden' reflections may be useful for high-resolution purposes. Detailed calculations of the necessary crystal orientations have been carried out for beryllium, magnesium and zinc. On the basis of somewhat arbitrary criteria the calculations show that these crystals can reflect a total of approximately 200 wavelengths in the range 0.7 to 4.9 Å (neutron energy 0.003 to 0.17 eV). Several orientations have been found at which mosaic crystals (with a sacrifice of angular resolution) should produce beams that are both intense and highly monochromatic.