The proportion of Friedel-related reflections present in the number of 'independent' reflections specified by the item _reflns.number_all. This proportion is calculated as the ratio: [N(Crystal class) - N(Laue symmetry)] / N(Laue symmetry) where, working from the DIFFRN_REFLN list, N(Crystal class) is the number of reflections obtained on averaging under the symmetry of the crystal class N(Laue symmetry) is the number of reflections obtained on averaging under the Laue symmetry. Examples: (a) For centrosymmetric structures, the value of _reflns.Friedel_coverage is necessarily equal to 0.0, as the crystal class is identical to the Laue symmetry. (b) For whole-sphere data for a crystal in the space group P1, _reflns.Friedel_coverage is equal to 1.0, as no reflection h k l is equivalent to -h -k -l in the crystal class and all Friedel pairs {h k l; -h -k -l} have been measured. (c) For whole-sphere data in space group Pmm2, _reflns.Friedel_coverage will be < 1.0 because although reflections h k l and -h -k -l are not equivalent when h k l indices are nonzero, they are when l=0. (d) For a crystal in space group Pmm2, measurements of the two inequivalent octants h >= 0, k >=0, l lead to the same value as in (c), whereas measurements of the two equivalent octants h >= 0, k, l >= 0 will lead to a zero value for _reflns.Friedel_coverage.