The process of discriminant analysis has been applied to major and trace elements in igneous and sedimentary rocks to seek to identify the original tectonic setting in which the rocks formed. A ‘training set’ of data from known environments is used to construct a discrimination diagram which is then used with data from unknown sources. Normally, the discrimination diagrams are based upon immobile trace element data and they have been applied predominantly to mafic igneous rocks, although there are also applications to felsic rocks and sediments. In the past, diagrams of this type have been used indiscriminately and here a robust approach is advocated for statistical analysis. Some of the diagrams presented are based upon elemental data, while others are based upon calculated discriminant functions and require some specific pre-calculation. Diagrams of this type are paradoxically accurate and at the same time geochemically opaque.