Friedel's law, named after Georges Friedel, is a property of Fourier transforms of real functions.
Given a real function , its Fourier transform
has the following properties.
where  is the complex conjugate of .
Centrosymmetric points  are called Friedel's pairs.
The squared amplitude () is centrosymmetric:
The phase   of  is antisymmetric:
Friedels law is used in X-ray diffraction, crystallography and scattering from real potential within the  Born approximation. Note that a [http://reference.iucr.org/dictionary/Twin_operation twin operation] ( Opération de maclage) is equivalent to an inversion centre and the intensities from the individuals are equivalent under Friedels law.