International
Tables for Crystallography Volume B Reciprocal space Edited by U. Shmueli © International Union of Crystallography 2010 
International Tables for Crystallography (2010). Vol. B, ch. 1.3, p. 35

The kernels of the Fourier transformations and satisfy the following identities:As a result the transformations and themselves have the following `conjugate symmetry' properties [where the notation of Section 1.3.2.2.2 will be used]:Therefore,
Conjugate symmetry is the basis of Friedel's law (Section 1.3.4.2.1.4) in crystallography.