Universal Principles of Design, Revised and Updated

1 The seminal work on similarity is “Untersuchungen zür Lehre von der Gestalt, II” [Laws of Organization in Perceptual Forms] by Max Wertheimer, Psychologische Forschung, 1923, vol. 4, p. 301–350, reprinted in A Source Book of Gestalt Psychology by Willis D. Ellis (ed.), Routledge & Kegan Paul, 1999, p. 71–88. See also Principles of Gestalt Psychology by Kurt Koffka, Harcourt Brace, 1935.

2 Note that a significant portion of the population is color blind, limiting the strategy of using color alone. Therefore, consider using an additional grouping strategy when using color.