Similarity — people perceive similar objects as the same or at least part of the same concept.

Proximity — things close to each other are perceived as part of the same group.

Continuity — you want your visitor’s gaze to move naturally from one element to another.

Closure — an object that’s not entirely closed is still perceived as a whole because people’s brains will always fill in the gaps.

Figure and ground — people simplify images into the main objects they’re looking at (the figure) and the other elements (background).