Key Takeaways from The Design of Everyday Things : Chapter 4

Knowing What to Do

Constraints, Discoverability and Feedback

Four Kinds of Constraints

• Physical Constraints
  e.g. A large peg cannot fit into a small hole.
• Semantic Constraints
  e.g. The purpose of the windshield is to protect the rider’s face, so it must be in front of the rider.
• Cultural
  e.g. Red is the culturally defined standard for a brake light, which is placed in the rear.
• Logical Constraints
  e.g. Suppose you take apart a leaking faucet to replace a washer, but when you put the faucet together again, you discover a part left over.

Constraints That Force the Desired Behavior

Forcing functions are a form of physical constraint: situations in which the actions are constrained so that failure at one stage prevents the next step from happening.

• Interlocks
  Microwave ovens and devices with interior exposure to high voltage use interlocks as forcing functions.
• Lock-ins
  Standard lock-ins exist on many computer applications, where any attempt to exit the application without saving work is prevented by a message prompt asking whether that is what is really wanted.
• Lock-outs
  The pin that prevents a fire extinguisher from being activated until it is removed is a lockout forcing function to prevent accidental discharge.

Conventions, Constraints, and Affordances

• Conventions are a special kind of cultural constraint.
• How does one go from the perception of an affordance to understanding the potential action? In many cases, through conventions.

Applying Affordances, Signifiers, and Constraints to Everyday Objects

Affordances, signifiers, mappings, and constraints can simplify our encounters with everyday objects.

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