The World Reference Base for Soil Resources (WRB) is the verbal basis to correlate national soil classification systems (NSCS); it also tends to be the specific language of soil science. It is unfeasible and hardly possible to build this scientific language by analogy with the usual literary language of people's communication. The first one requires strict formalization and imposes definite limitations on its semantic content. Vague intuitive classification concepts of similarity, identity, or indistinctiveness must be precisely and qualitatively identified in terms of the degree of equivalency or in more general terms of the degree of tolerance. In turn, the latter notions can be described in terms of reflection, symmetry, and transitiveness. Mathematics is the general language of science to give "different things one name," and the formal logics is the language of statements and judgments.Soil classification describes the structure of soil taxa in the space of soil attributes (properties) forming the information basis of soil classification. Soil taxa of particular NSCS are being subjectively transposed into the WRB; taken together, these national soil taxa compose the whole set of soil units to be classified (Fig. 1). The tolerance level of particular NSCS can be unambiguously determined if all the NSCS have a common information base. Classification can be called a system if it displays some emergent features, i.e., if it presents more than a sum of its elements. In the case of soil classification, this might be its hierarchical structure visualized in the form of a dendrogram. A comprehensive structure of soil information base with 64 major attributes was suggested. Using this information base, the analysis of soil types distinguished in the Russian soil classification, soil orders in Soil Taxonomy, and major soil groups of the FAO system has been done to find the degree of similarity between them (Fig. 2). The particular values of soil attributes were assessed in a binary system ("present" or "not present"). This method makes it possible to estimate quantitatively the degree of similarity between the national soil classification taxa in percent. The hierarchical structure of the resulting dendrogram integrating three different NSCS is open to argument, but it can be clearly and unambiguously displayed. The analysis of the list of major soil attributes suggests that their number can be reduced from 64 to 44.The choice of soil attributes to be put in the common information base requires expert judgment. First of all, those attributes (diagnostic features, etc.) should be included that are proposed by all the experts. Then, those that are proposed by most of the experts, and so on. The weights of particular attributes and qualitative gradations inside them are also to be indicated by experts.