Design AtlasOne of the perceptual principles Max Wertheimer and other early-20th-century Gestalt psychologists established; Stephen Palmer named and formalized "common region" in the 1990s. It resembles the law of proximity (nearby elements read as a group), but a shared boundary alone can override a lack of proximity.
Card UI is the clearest example — a title, image, and button can sit somewhat apart, but a shared card border still reads them as one unit. Without that boundary, users struggle to tell whether a button belongs to this card or the next one.
A scatters six dots with no boundary, so grouping is ambiguous; B splits the same dots into two boxes and reads instantly as two groups.
When to use
Use this when deciding whether to draw a card or section boundary. If proximity alone isn't reading as a group, a faint background or border often fixes it.