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consistency

Apply the principle of Consistency — making similar things look and behave similarly so users can transfer learning across the interface. Use when designing component libraries, defining naming conventions, choosing whether to invent or borrow patterns, auditing across surfaces (web, mobile, email, support docs), or evaluating whether two features should share or diverge in their interaction model. Strong consistency reduces cognitive load and learnability cost; aggressive consistency can flatten meaningful differences. There are four kinds — aesthetic, functional, internal, external — each with different applications.

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# Consistency

> **Definition.** Consistency is the principle that similar things should look and behave similarly, and that different things should look and behave differently. When two parts of a system serve the same function, they should be expressed the same way, so the user's learning about one transfers immediately to the other. When two parts serve different functions, they should be visually and behaviorally distinct, so the user is not misled into expecting the same behavior.

The principle sounds simple and is, in practice, one of the hardest to apply well. The hard part is judging *what counts as similar*. Two buttons that share a visual style but trigger different kinds of actions are misleadingly consistent — they tell the user "we work the same way" when they don't. Two equivalents in the system that are styled differently for no good reason are inconsistent — they force the user to learn the same lesson twice. The skill is calibration: enough consistency to support transfer, enough variation to honor genuine differences.

The classic taxonomy, due to Lidwell, Holden, and Butler, distinguishes four kinds of consistency.

**Aesthetic consistency.** Visual style, typography, color, spacing, iconography. The interface looks like it belongs to one product, drawn from one design system. Aesthetic consistency builds brand recognition and signals craft. Its absence — components that obviously came from different libraries, typography that varies arbitrarily — signals that the team didn't coordinate.

**Functional consistency.** Equivalent operations work the same way. The "save" gesture saves in every screen; the "delete" affordance deletes in every list; the "back" gesture goes back everywhere. Functional consistency lets users learn an interaction once and apply it throughout the product. Its absence creates "exception fatigue" — the user can't predict what any control will do without reading.

**Internal consistency.** Within the product, similar things are similar. This combines aesthetic and functional consistency at the level of "we have one design system and we apply it." Internal consistency is what users notice as "this product feels coherent."