A constraint restricts the performance of certain actions. A door that blocks passage into an unsafe area is a constraint. Constraints are an important method for error proofing because they can limit the actions that result in errors. They do not just make information available in the external world but make the information available within the product or system itself. To be effective, constraints should be visible and easy to understand.

Constraints can be built into a process so that accidental stopping or an unwanted action that will result in an error can be prevented. Constraints can also be used to make sure that the steps performed in a process or when using a product are accomplished in the correct sequence.