Constraints

Limitations built into a design that restrict possible actions and guide people toward correct use. Norman identifies four kinds: physical, logical, semantic, and cultural.

Why it Matters

Constraints shrink the space of what a person must figure out, often making the correct action the only possible one. They let people act correctly without prior knowledge or instruction.

Signals

• Many ways to do something wrong; reliance on instructions to avoid mistakes.
• Parts that can be assembled in the wrong order or orientation.

Benefits

Fewer errors, less to learn, and operation that is self-evident.

Risks

Too few constraints (anything goes, easy to err) or arbitrary ones that frustrate legitimate use.

Tensions

Constraints that prevent error also block flexibility and expert shortcuts: restriction competes with freedom.

Examples

A connector that only fits one way (physical); a form that disables submit until input is valid (logical); a red-hot, blue-cold tap convention (cultural).