On a basis of more than 100 complete T-x-diagrams of systems involving nematic liquid crystals (NLC) the dependence for a width of a nematic solution region from the component mesomorphism type was revealed. A classification for T-x-diagrams of binary systems was elaborated using this relationship and the principles known from the geometry of systems with non-mesogen substances. The classification has include the systems of two monotropic NLC, enantiotropic NLC-non-mesogen, monotropic-enantiotropic NLC. 8 types of systems have been predicted and first investigated by the use of the classification.