Law of Similarity
Elements that look alike are often read as part of the same group.
A shared look suggests a shared role.
A page has six buttons that do the same kind of job. Five share one shape and color, while the sixth looks unrelated. People pause at the odd one because the visual pattern has changed, even before they read its label.
Origin
Wertheimer described similarity in 1923. With spacing held constant, changing the color of selected dots changed how they grouped. Modern research continues to study similarity in color, size, orientation, and other visible properties.
Give controls with the same role a consistent appearance and behavior. People should be able to reuse what they learned from the first one.
Make different roles visibly different. A destructive action should not look identical to an ordinary next step.
Do not rely on color alone. Pair it with a label, icon, shape, or position so the grouping still works for people who cannot distinguish the colors.
Use similarity with proximity. Matching elements scattered across a page may look related, but their distance can still make the relationship harder to follow.
Digital systems
A Consistent Button Set
When primary buttons share the same shape, color, and label style, people can recognize their role across screens. A different treatment should signal a different kind of action.
Physical signals
Team Uniforms
Players wearing the same uniform are read as one team even when they spread across the field. Shared color and pattern provide the grouping cue.
When the same kind of control looks different from screen to screen, people have to inspect it again. When different actions look identical, the interface hides an important distinction.
Use a consistent treatment for shared roles and a clear change for different roles. Support the pattern with labels and placement instead of asking appearance to carry every meaning alone.