A complete figure can be seen even when parts of it are missing, due to the figure(s) surrounding it.
The figure leads the eye to follow a certain direction or path.
A figure is clearly placed prominently against a solid background (there is a clear foreground and background).
The eye associates certain figures into groups, due to their close positions (their proximity).
Two or more figures are shown to be so similar that even though they may have slight differences, the viewer can still perceive them to be in the same group.