Structure of Subjects: The structure of subjects can be viewed from the following angles-
a) Dichotomy: Dichotomy refers to a division into two. This is also referred to as binary classification. In this system of divisions, the subjects are divided into two broad categories, each broad category are again divided into two sub categories, and so on. Immanuel Kant gave a dichotomous picture of the entire universe of subjects; however dichotomy is insufficient for designing a scheme of classification for the existing universe of subjects. “Tree of Porphyry” is a schematic representation of dichotomy.
b) Decachotomy: Decachotomy refers to a division into ten. Melvil Dewey Classification is the best example of this kind of division. Dewey divided the field of knowledge into nine main classes and tenth class was formed for general documents not belonging to any of the main classes. This process of division into ten at each stage is continued until the required subdivisions have been obtained. From the point of nature of growth and development of knowledge it is unrealistic to bind the universe of subjects to a decachotomy, because it grows in different directions and at different stages.
c) Polychotomy: Polychotomy refers to a division into many. In 1893 C. A. Cutter in Expansive Classification introduced polychotomy in a limited way by stipulating the number of division (at each stage of division) to be 24. But the restriction to 24, by the middle of twentieth century was found to be an impediment. The lesson is that the number of division to be incorporated at a given stage of division should not be predetermined.
d) Proliferation: It is not possible to predict the maximum number of division to be provided for in a particular array or stage of division, because various are the ways in which the universe of subjects gets proliferated. The extensive proliferation creates problems for the designers of schemes of classification.