Proximity
the tendency of individual elements to be associated more strongly with nearby elements than with those farther away.

Similarity
elements that share common visual variables such as color, will be more associated than those that do not.

Continuity
the preference for unbroken figures with the simplest possible explanation.

Closure
the tendency to interpret forms as complete whole figures even though some pieces are missing. This example also contains figure-ground reversal.

Area
the smaller of two overlapping figures will be interpreted as figure while the larger is interpreted as ground.

Symmetry
grouping based on properties of the entire form instead of its parts. This looks more like one object than three.