On a clear disk you can seek forever.
Whether a product is a handheld two-way radio or a computer's desktop, it is not always clear what functions are available, what they do, or how they are accessed. You should be able to use your senses to easily discover both what abilities are available and how they are to be operated.
An interface feature is visible if it either is currently accessible to a human sense organ usually the eyes, although this discussion applies also to other sensory modalities or was so recently perceived that it has not yet faded from short-term memory. If a feature is not visible, we say that it is invisible. For an interface to work well, "[j]ust the right things have to be visible: to indicate what parts operate and how, to indicate how the user is to interact with the device. Visibility indicates the mapping between intended actions and actual operations" (Norman 1988, p. 8). If an interface forces you to memorize the fact that a feature exists, that feature is invisible. If you are forced to root around in the interface until, by luck and perseverance, you reach a sequence of actions that activates a feature, such a feature is not visible. If you have to use a help system to discover how to perform an operation, the methods for invoking that operation are invisible. Many computer games are, essentially, undocumented interfaces in which the controls, or their mapping to desired effects, are invisible. Add documentation and these games become trivial. Most people do not want to play games when they are trying to get work done. It is up to the designer of an interface to make every feature of a product visible.
 This book, The Psychology of Everyday Things, is not to be missed.
In designing to accommodate visibility, each function and the method of operating it would be apparent to most people in the culture for which it is intended by merely looking at it. A control that has this attribute has come to be called an affordance (Norman 1998, p. 123). "Affordances provide strong clues to the operations of things.... Knobs are for turning. Slots are for inserting things into. Balls are for throwing or bouncing" (Norman 1988, p. 9). If you, as a designer, put a knob such as is used in volume controls on a product, people will attempt to turn that knob. Put on something that looks like a pushbutton, and people will push it. Whether a feature is or is not an affordance depends on the experience and the background of the people who will be using the product and the context in which the feature appears.
In analyzing interfaces, we should always ask how the user knows that an action is possible, and we should always require that each visible feature provide a recognizable affordance. Icons are often considered the epitome of visible affordances, but they do not always serve this function, as we discuss in Section 6-3.
Visibility is more than just detectable existence. An object may be visible in the sense that it is there, but it might be too small to be noticed, or it may have too little contrast to be readily distinguished from the background. Optimizing the perceptual qualities of an interface is an important ergonomic consideration, but the concern here is with the cognitive properties of interfaces.
Affordance Avoidance: BART
It is the interface to its ticket machines that brings Bay Area Rapid Transit (BART) into this book. The machines at a local station are the result of the transit authority's ignorance of 25 years of experience with the old ticket-dispensing robots. As I stand in line at one of the two machines not presently out of service, I watch person after person baffled by the machine's layout. Although experienced with the machines myself, I still fumble at the unnatural sequence of operations that they require.
The obvious way to start the interaction, and what nearly everybody whom I've observed tries at first, is to put in money or an old ticket, which is a money-equivalent. That's how vending machines work: Put in money, make choice, get stuff. It is a cultural paradigm.
The BART ticket-vending machines must have been designed on another planet. First, you choose between BART and BART-MUNI buttons. (A pamphlet, available nowhere in the vicinity, explains the subtle difference.) These buttons are at the top of the wall of the machine face: a reasonable place to start but an unreasonable transaction with which to start. The slots into which you then put your ticket (to the left), coins (in the middle), or bills (lower right) are widely separated instead of being clustered. The LCD display is up high at top center, where short people cannot easily read it; in any case, it is far enough from the rest of the interface that many people do not notice it at all. Maybe it's good they do not see it, in that its response time is so slow that people who do watch it either repeat their actions in hope of forcing a response or stand there wondering whether the machine is broken, which it often is.
Then there's the "correction" button, labeled as though it is to be used in case you've made a mistake that has to be corrected. In fact, you do not use this button for correcting a mistake: You use it if you want the ticket to be issued for less than the total amount of money that you have put in. That is, it simply tells the machine you want some change back.
Another indication that the new machines are badly designed is the bright, colorful, large numbers "1, 2, 3, 4" and the big arrows presumably placed to guide you. If the front of the machine were well laid out, you wouldn't need crutches. The numbers do not help tremendously anyway, because you must remember what step you are executing (say, step 2) while you put in your money ($5), knowing that you only want to get a $3 ticket so you must hit the correction button in the next step (step 3), keeping in mind that you'll want to count your change ($2) that will come in the form of quarters (eight of them). Hurry! You can hear the 7:06 train coming! What step did you say you were on? When you find a computer interface that requires bright colors and myriad explanatory legends to guide you, you can guess that the design has gone astray.
Here is one way in which the BART machines could work. Remember, however, that were you consulting to BART, you would request user testing of your proposed design to verify that people will use it as you have predicted.
On the left because English and Spanish are written from left to right is the slot for old tickets, a horizontal slot for bills, and a small vertical slot for coins, grouped together and well labeled. The display located at average eye level reads: "Put in money OR your old BART ticket OR both." As you put in each, the total value accrued appears immediately in the LCD display with this message:
You now have put in $4.55.
If you want a ticket for more than $4.55, put in more money or another old ticket
If you want a ticket for less than $4.55 press:
Change Ticket Amount
If you want a ticket for $4.55, press one of these buttons:
Issue BART Ticket
Issue MUNI-BART Ticket
If the user selects "Change Ticket Amount," he is asked to type in the amount for which he wants the ticket issued and to press the large ENTER button that is under the numeric key pad. The user is returned to the display that shows the current total and asks him to increase it, decrease it, or OK it. When the ticket is issued, any change due the user is returned. For the experienced traveler with the right amount of money in hand, this interface reduces six actions to these two: put in money and then press an Issue-Ticket button.