The plastic number is based on the results of tests in perceptual psychology. These tests were searching for instinctually (easily) understood relations between the sizes of objects (including "belonging to a group" of some size), as opposed to proportions that are "preferred" or "beautiful." The proportions resulting from these tests were not pegged to any particular unit, making them rather difficult to use in a design situation. The creator of the system (Dom. Hans van der Laan), played with the sets of possible proportions until he came to a set that came strikingly close to those of the Pythagorean divisions of the octave.
Van der Laan limited the range of scales over which the plastic number proportions are used. This was because he found that proportions that went beyond that limited set ceased to have any recognisable relationship between them. Because of this limitation, Van der Laan was able to provide exhaustive collections of those shapes suitable for use with the plastic number.
Document Name: tc.genorg.org.sizer.plastic.htm
Copyright (c) 1997 -- Mike Fletcher
Reproduction for other than personal use prohibited without express written permission from the author.