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1.6 More on usability: design and usability principles 25

sion so that the menus are structured in the way I think they should be, but this all
takes considerable time (especially when I use different machines at work, home,
and when travelling).
Another problem with consistency is determining what aspect of an interface
to make consistent with what else. There are often many choices, some of which
can be inconsistent with other aspects of the interface or ways of carrying out ac-
tions. Consider the design problem of developing a mechanism to let users lock
their files on a shared server. Should the designer try to design it to be consistent
with the way people lock things in the outside world (called external consistency)
or with the way they lock objects in the existing system (called internal consis-
tency)? However, there are many different ways of locking objects in the physical
world (e.g., placing in a safe, using a padlock, using a key, using a child safety lock),
just as there are different ways of locking electronically (e.g., using PIN numbers,
passwords, permissions, moving the physical switches on floppy disks). The prob-
lem facing designers is knowing which one to be consistent with.

Ahbrdance is a term used to refer to an attribute of an object that allows people
to know how to use it. For example, a mouse button invites pushing (in so doing ac-
tivating clicking) by the way it is physically constrained in its plastic shell. At a very
simple level, to afford means "to give a clue" (Norman, 1988). When the affor-
dances of a physical object are perceptually obvious it is easy to know how to inter-
act with it. For example, a door handle affords pulling, a cup handle affords
grasping, and a mouse button affords pushing. Norman introduced this concept in
the late '80s in his discussion of the design of everyday objects. Since then, it has
been much popularized, being used to describe how interface objects should be de-
signed so that they make obvious what can be done to them. For example, graphi-
cal elements like buttons, icons, links, and scroll bars are talked about with respect
to how to make it appear obvious how they should be used: icons should be de-
signed to afford clicking, scroll bars to afford moving up and down, buttons to af-
ford pushing.
Unfortunately, the term affordance has become rather a catch-all phrase, los-
ing much of its potency as a design principle. Norman (1999), who was largely re-
sponsible for originally promoting the concept in his book The Design of Everyday
Things (1988), now despairs at the way it has come to be used in common parlance:

"Zput an affordance there, " a participant would say, "I wonder if the object affords
clicking. . . " affordances this, affordances that. And no data, just opinion. Yikes! What
had I unleashed upon the world? Norman's (1999) reaction to a recent CHI-Web
discussion.
He has since tried to clarify his argument about the utility of the concept by saying
there are two kinds of affordance: perceived and real. Physical objects are said to
have real affordances, like grasping, that are perceptually obvious and do not have to
be learned. In contrast, user interfaces that are screen-based are virtual and do not
have these kinds of real affordances. Using this distinction, he argues that it does not
make sense to try to design for real affordances at the interface--except when design-
ing physical devices, like control consoles, where affordances like pulling and press-
ing are helpful in guiding the user to know what to do. Alternatively, screen-based