From: The World In Your Head by Steven Lehar

Figure 3.1 Examples of some of Wertheimer’s laws of perceptual grouping: (A) similarity, (B) proximity, (C) good continuation, (D) closure, (E) symmetry, and (F) periodicity. Although these groupings are experienced preattentively, it is difficult to devise explicit rules that predict the grouping percept for arbitrary figures.