laws & principles
Gestalt: similarity
What is the Gestalt law of similarity?
The Gestalt law of similarity tells us we mentally group items that share visual traits - like colour, shape, or size - and perceive them as belonging together, even when spaced apart. Similarity alone suggests they form a set.
Also known as: law of similarity, similarity principle
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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.
We group visually similar items - same colour, shape, or size - and perceive them as a set, even when scattered. Give related elements consistent styling; likeness alone creates grouping, without needing lines or extra spacing. You can see and try this in action on this page.
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 is generally the gentlest of the grouping cues, though - spacing usually wins, so a shared colour rarely pulls items together against clear proximity or an enclosing region. Lean on it to reinforce groupings, not to fight them. 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.