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The term `direct methods' is applied to that class of methods which seek directly to solve the phase problem by the use of phase relationships based on the observed intensities.

The object of this pamphlet is to familiarize the reader with the phase relationships used in Direct Methods, and to explain why they work and how they are used in practice. Some prior knowledge of the phase problem, the structure-factor equation and the application of Fourier theory in crystal-structure analysis is assumed.

**Copyright © 1984, 1998 International Union of
Crystallography**