Chapter four is all about knowledge in the world and how designers can improve affordances, signifiers, constraints, and mapping.
Norman begins by asking: "How do we determine how to operate something we have never seen before?" The answer? We must "combine knowledge in the head and knowledge in the world".
In this case, knowledge in the world means affordances, signifiers, constraints, and natural mapping, whereas knowledge in the head is based mostly on cultural, semantic, and logical constraints, conceptual models, and previous experiences.
Norman then introduces us to four different kinds of constraints:
1. Physical limitations
Physical limitations rely on the properties of the physical world. They limit the number of interactions that are possible between human and object. 
No training is needed to perceive them, but they are more effective and useful if they are easy to see and interpret, for else they must rely on the principle of trial and error
An example given by Norman is the classic plug-in component,  most of which are clearly constrained in the way in which they can be plugged in. However, it becomes more difficult when the constraint is less perceptible, such as with the famously frustrating USB port. 
2. Cultural Constraints
Each culture has a set of allowable actions for each situation, based on learned knowledge: Rules, schemas, and “scripts” guide behavior in a culturally appropriate fashion.
As these constraints differ from one culture to another, they can be the cause of many misunderstandings and accidental insults. Also, they are likely to change over time as cultures develop and merge.
3. Semantic Constraints
Semantic constraints are based on the interpretation of signs and symbols, which can also be culturally significant. This interpretation relies upon our knowledge of the situation and of the world and it can change over time and from place to place.
4. Logical Constraints
Logical constraints can be found when one action makes logically more sense than another, based on the situation.
An example given by norman is natural mapping: "The logical relationship between the spatial layout of controls and objects that they control". This type of natural mapping is unfortunately rarely used and therefore can lead to more confusion than actually be useful. Norman uses the example of light switches and how confusing they can oftentimes be. 

Lastly, Norman speaks about Cultural Norms, Conventions, and Standards and how they can encourage or discourage certain behavior, as violating them shows someone to be an outsider and can be the cause of ostracization. Such norms and conventions are usually related to behavior in social situations but can sometimes be decreed into law. 
Applying Affordances, Signifiers, and Constraints
Norman explains how we need visual signals (or signifiers) to signify both the appropriate action and its location (WHAT can be done and WHERE can I do it?). 
Signifiers, in part driven by physical constraints, also help clarify HOW to act. Visible affordances that also act as physical constraints are the ideal way to narrow down WHAT can be done and WHERE and HOW to do it. 
Very important is the action of constraining improper behavior to avoid errors! Too much focus on aesthetics can sometimes blind both designers and users from the usability issues of a product. Appearances deceive!
Constraints that force desired behavior = Forcing Functions
Forcing functions are a form of strong physical constraint, where failure at one stage prevents the next stage from happening. They are put in place to prevent inappropriate (or dangerous) behavior. Norman gives three examples of forcing functions: 
1. Interlocks
An interlock forces a proper sequence of behavior, for example, a microwave oven that won’t start until the door is closed. This also includes the so-called "Dead man’s switch" which prevents operation if the operator is incapacitated. 
2. Lock-ins
A lock-in keeps an operation active and prevents premature interruption, for example, the prompt to save your work before closing a file on a computer.
3. Lock-outs
Lock-outs prevent an event from occurring, usually used for safety reasons. They can be a nuisance for normal usage, up to the point where people may disable them out of annoyance. So good design must minimize nuisance while maximizing safety.
Activity-centered controls map controls to activities, not functions. They need to be carefully selected to match the actual requirements of real situations.
An example is given by Norman: Choosing to control lights, sounds, and video projection in a lecture hall not by type but according to the situation. 
Using Sound as Signifiers
Sounds give us information about the source and convey something about our interaction with it. They are highly important as they can break through our concentration even when our eyes are focused elsewhere, so they are especially useful for warnings or error messages
It can be very tricky to design a sound to be subtle enough not to be annoying but still strong enough to draw attention when needed. 

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