Elements that resemble one another are perceived as belonging to the same group.
Wertheimer's 1923 principle. In interfaces it competes with proximity, and when the two conflict, proximity usually wins — which is why inconsistent spacing overrides consistent styling.
In practice
- A table where every row is styled identically is read as one block, however carefully the columns are labelled.
- Two buttons of the same colour are assumed to do similar things, and the destructive one is pressed.