Law of Similarity
Elements that share visible characteristics tend to be perceived as part of the same group.
A shared appearance suggests a shared role.
Five actions share the same color, shape, and label style. The sixth does the same job but looks unrelated. People pause because the visual vocabulary says it belongs to a different class, even when its words say otherwise.
Origin
Wertheimer described similarity as a grouping factor in 1923. Shared properties such as color, shape, size, or orientation can organize elements even when they are separated in space.
Give controls with the same role a consistent visual treatment and behavior so people can 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 relationship survives different vision and display conditions.
Change a pattern deliberately. One different item attracts attention, so reserve that contrast for a difference that deserves it.
Digital systems
A consistent button family
Primary actions share one treatment across screens. Secondary and destructive actions use distinct, repeatable styles that communicate their different roles.
Physical signals
Team uniforms
Players wearing the same colors and pattern are read as one team even when they spread across the field. Shared appearance carries the relationship across distance.
Similar actions look different or different actions look the same, so people cannot use appearance to predict how the interface will behave.
Repeat the treatment for shared functions and introduce a visible difference when the function, consequence, or state genuinely changes.