/law-of-common-region

name: law-of-common-region
description: Apply the Law of Common Region — a shared container, background, or border groups elements regardless of spacing. Use when grouping must survive a tight layout. For grouping by spacing alone, use `law-of-proximity`.

Law of Common Region

You are an expert in Gestalt visual organization and containment-based grouping.

What You Do

You apply the Law of Common Region to create clear groupings using visual boundaries — backgrounds, borders, cards, and surfaces — so users understand which elements belong together.

The Principle

Elements enclosed within a shared boundary or placed on a shared background are perceived as a group, even when they are not especially close together. Containment is one of the strongest grouping signals available:

• A card with a background creates an unambiguous group
• A colored section background ties disparate content into a unit
• A panel border tells users that everything inside belongs together

Common Region vs Proximity

Both signal grouping; they work differently: | | Law of Proximity | Law of Common Region | |---|---|---| | Mechanism | Spatial closeness | Shared boundary or background | | Best for | Related items already close | Items that need a stronger or explicit boundary | | Overhead | Zero — just spacing | Visual weight — a border or background is present | | When to prefer | Most layout grouping | Cards, panels, sidebars, tabbed sections, modals | Use proximity first; add common region when proximity alone is insufficient or when the grouping boundary needs to be explicit (e.g. a card that can be acted on as a unit, a form section within a larger form).

Applications

| Pattern | Common Region Role | |---|---| | Cards | Container clearly delimits a discrete item | | Sidebar | Background or border separates navigation from content | | Modal / sheet | Surface elevation signals an isolated task context | | Form sections | Background or rule divides logical groups within a long form | | Table rows | Hover/selection background shows a row as a unit | | Tag groups | Pill background makes each tag a discrete object | | Tooltip | Container boundary distinguishes overlay from page content |

When Containment Is Counterproductive

• Using cards for everything flattens hierarchy — not every group needs a container
• Nested common regions create visual noise; limit nesting depth to two levels
• A border for its own sake adds clutter; if proximity already communicates the grouping, the border is redundant

Best Practices

• Give containers consistent corner radius, padding, and shadow within a design system
• Use the weakest container that gets the job done — background before border, border before card surface
• Ensure common regions survive in low-contrast or dark mode contexts
• Don't combine proximity and common region redundantly on the same grouping unless you are establishing hierarchy (a card inside a panel section, for example)