laws & principles
Gestalt: similarity
What is the Gestalt law of similarity?
The Gestalt law of similarity says we mentally group things that look alike - same colour, shape or size - and read them as belonging together, even when they're scattered. Likeness is enough to imply "these are a set".
Also known as: law of similarity, similarity principle
The demo
A plain grid of dots. Switch on similarity and watch groups appear - columns, then rows - created by nothing but shared colour. No lines, no boxes; your eye does it.
Switch on similarity and see groups form from colour alone.
What this demo shows (text version)
A six-by-six grid of identical dots. In uniform mode every dot is the same colour, so the grid reads as one undifferentiated block with no grouping.
Switching to "by column" tints alternate columns a different colour, and the eye immediately perceives vertical groups; "by row" does the same horizontally. No lines, boxes or spacing change - the grouping comes entirely from shared colour. That is the Gestalt law of similarity: things that look alike are read as belonging together, which also means things that look alike are assumed to behave alike.
As a uniform grid it was just dots; tint every other column and you suddenly saw columns, groups, structure - without a single line or box being drawn. Your eye did the grouping on its own, purely from likeness. That's similarity doing your information design for free.
Similarity is how you signal "same kind" without borders or labels: links share a colour, primary buttons share a style, required fields share a marker. It also works against proximity - matching colour can bind items that sit far apart, and can override spacing - so wield it deliberately. See its sibling, [Gestalt: proximity](/entries/gestalt-proximity/).
The flip side is the warning: if two things look alike, people will assume they behave alike. Style a non-clickable label like your links and someone will click it; give a destructive button the same look as a safe one and the similarity is a trap. Consistent appearance is a promise of consistent meaning - keep it.