Exact formal solutions of the transport equations which govern the flow of current in a mosaic crystal set at a Bragg condition are given. These solutions are applied to a crystal cut in the shape of a parallelepiped. A formula is derived for the extinction coefficient for this crystal shape. We call this result the ``AB‐extinction formula''. It will provide the experimentalist with an approximate method to correct for anisotropic extinction resulting from a crystal shape anisotropy. It is suggested that the exact results of the dynamical diffraction theory for slabs should be used to correct for primary extinction. The mathematical and conceptual errors in the theory given by Zachariasen are pointed out.