[en] The in-plane resolution of a diffraction apparatus is discussed in the Gaussian approximation. This approximation is generally adequate to find half-widths of the resolution function, but the detailed resolution line-shape may differ from a Gaussian line-shape for instance in using perfect crystals as monochromator or analyzer. In combining the contributions from different elements in the apparatus to the resolution function the method of conjugate diameters is useful. The resulting resolution widths are not given in explicit formulae but by means of a few simple subroutines in a computer program, e.g. transforming a set of conjugate diameters to another set with a prescribed direction of one diameter, the method seems to be readily applicable to a variety of instruments. (Auth.)