Overview of DeCS's tree structure DeCS follows the tradition of the classification systems and their respective subject heading lists which have been transformed into specialized vocabularies without, however, changing the structures of the classification systems from which they originated. Its tree structure is well-grounded in the division of knowledge in decimal classes and subclasses respecting their conceptual and semantic relationships, and its terms are presented in a hybrid structure of pre- and post-coordination. See also the qualifier hierarchical tree. When evaluating the proportionality of terms in its contents, we find some interesting numbers. From the aproximately 182.316 DeCS terms (2008 version): 25.6% refer to chemical compounds and drugs, both exogenous and endogenous; 21.6% of the total are the anatomical terms, the organisms and the biological sciences; the terms referring to disease, a very comprehensive category, represent 12.7% of the total; other areas such as techniques, equipment, allied sciences and geographic areas together represent 14,6%; we emphasize still the areas of Public Health, Homeopathy, Environmental Health, and Science and Health which represent each 10.3%, 5.8%, 2.5%, and 0.6% of the total of terms, respectfully. These last four categories were especially developed to better represent the literature generated in the countries of the region.