Summary: In visual design, elements within the same boundary are perceived as related.
Gestalt principles guide how people visually perceive the world — including digital interfaces. Specifically, these principles explain how people decide whether several individual elements are part of the same group and, thus, are related in some way. This knowledge helps them understand and interact with the world in general, and also applies to controls and content on screens.
The original set of Gestalt principles was discovered in the first half of the twentieth century and includes proximity, similarity, and closure. Later research at the end of the twentieth century added a few more grouping principles to the list discovered initially by the Gestalt psychologists. Among these, perhaps the most relevant for UX is that of common region.