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 Five Planes, and Desargues  A note:  Though I would ordinarily deprecate doing so, here, simply for convenience, I use the word, "common", for an element that is the incidence of two elements.  The word can be quite wrongly construed to mean that the element-of-incidence is part of both incident elements, which it never is, because elements do not have parts. Consider the incidence of five planes, seen as a group of three (3-p), and a group of two (2-p). Especially note that the planes of 3-p have a common point, O, and collectively, three common lines, and the planes of 2-p have a line in common. The whole assembly is a “prismatic cone” - or a “conical prism” - having two, triangular cross-sections, made by the two planes (not depicted on the left) of group 2-p — their line in common is the one drawn across the top of the sketch. The 3-p planes - form the “walls”, their three common lines the “corners”,and their common point the vertex, O, - of the “cone”.The drawings are given as an aid to the imagination – and in the hope of making clear that the Desargues Configuration [*], which this is, is the complete incidence of five planes. Another view of the Five Planes - This always happens when and where five planes congregate.