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1.6 More on usability: design and usability principles 23

Figure 1.9 (a) Natural mapping between rewind, play, and fast forward on a tape recorder
device. (b) An alternative arbitrary mapping.

trigger something else to happen. Making actions and their effects obvious enables
people to logically deduce what further actions are required. Disabling menu op-
tions when not appropriate for the task in hand provides logical constraining. Jt al-
lows users to reason why (or why not) they have been designed this way and what
options are available.
Cultural constraints rely on learned conventions, like the use of red for warn-
ing, the use of certain kinds of audio signals for danger, and the use of the smiley
face to represent happy emotions. Most cultural constraints are arbitrary in the
sense that their relationship with what is being represented is abstract, and could
have equally evolved to be represented in another form (e.g., the use of yellow in-
stead of red for warning). Accordingly, they have to be learned. Once learned and
accepted by a cultural group, they become universally accepted conventions. Two
universally accepted interface conventions are the use of windowing for display-
ing information and the use of icons on the desktop to represent operations and
documents.

Mapping This refers to the relationship between controls and their effects in the
world. Nearly all artifacts need some kind of mapping between controls and effects,
whether it is a flashlight, car, power plant, or cockpit. An example of a good map-
ping between control and effect is the up and down arrows used to represent the up
and down movement of the cursor, respectively, on a computer keyboard. The
mapping of the relative position of controls and their effects is also important. Con-
sider the various musical playing devices (e.g., MP3, CD player, tape recorder).
How are the controls of playing, rewinding, and fast forward mapped onto the de-
sired effects? They usually follow a common convention of providing a sequence of
buttons, with the play button in the middle, the rewind button on the left and the
fast-forward on the right. This configuration maps directly onto the directionality
of the actions (see Figure 1.9a). Imagine how difficult it would be if the mappings in
Figure 1.9b were used. Look at Figure 1.10 and determine from the various map-
pings which is good and which would cause problems to the person using it.

Figure 1.10 Four possible combinations of arrow-key mappings. Which is the most natural
mapping?