The known structure of 21 industrial gamma-quinacridone pigments served as a basis for the measurement of powder patterns with and without fluorophlogopite by a Bragg-Brentano diffractometer. Calibration and cell refinement were used to determine the cell parameters and to fit the profiles of the intensities. The unit cells vary systematically and the importance of the short b-axis is revealed. Averaging of all data leads to idealized unit cells. The corresponding criss-cross angles, tau(c), of the 21 pigments vary in a systematic way. It is possible to determine the relative frequency of the positions of the molecules inside the unit cell. The criss-cross angle of gamma-quinacridone increases from 2 x 26.0 degrees (hydrogen-bond : 0.266-0.268 nm) to 2 x 28.0 degrees (hydrogen-bond : 0.268-0.274 nm). The pigments are understood and described as physical mixtures of crystallographically similar structures. The colour of these pigments is one of the most important industrial properties. It varies from reddish purple to yellowish purple apparently depending on the structure. The difference between the gamma- and gamma’-form is described. Alpha-quinacridone is a special form of gamma-quinacridone and fits perfectly in the system of the latter. Evaluating the criss-cross angle of all known quinacridones, it is possible to classify their structure and colour, which are attributed to the cooperative effect of hydrogen-bonds and double bonds in the lattice (pi-sigma-correlation).