The APUPA bell curve. Ranganathan’s visual pattern for knowledge organization

Among many clever innovations, the pioneer of faceted classifications, S.R. Ranganathan, introduced a structural model called APUPA (Alien-Penumbral-Umbral-Penumbral-Alien). This model can be visualized in the form of a bell curve. APUPA is commonly cited as an ideal shape resulting from the arrangement of works according to the principles of faceted classification, which should enable a reader browsing shelves to visually find "his book" as located in the middle of a sequence of related subjects on both its left and its right. However, we show how this is but one application of a more general model of thought recurring throughout Ranganathan’s works. Main classes order of Colon Classification (CC), and anteriorizing common isolates, are two more features where the bell curve model can be observed. We analyze the presence of this concept in Ranganathan’s work through excerpts from a variety of his published books. The concept is compared with models adopted in the work of other classificationists, and with various ideas available in scientific literature, including Bernoulli numbers and the mathematical theory of communication. It can be concluded that the bell curve pattern is yet another facet of the deep originality of Ranganathan’s complex and rich classificatory thought.