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A method of measuring the difference between the lattice parameter of a region of an unknown sample and that of a region of a standard reference crystal to a sensitivity of 1 part in 109 is presented. Problems inherent in multiple-beam arrangements due to sample strains and non-uniformity have been overcome by the use of a new double-source arrangement in which the two X-ray beams sample the same spot on a crystal under study. Ways of identifying and preventing errors from significant mechanical and thermal effects arising in the sensitivity region explored are indicated.