Universal Principlesof Design

William LidwellKritina Holden

Ji l l Butler

125 Ways to Enhance Usability,Influence Perception, Increase Appeal, Make BetterDesign Decisions, and Teach through Design

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AffordanceA property in which the physical characteristics of anobiect or environnrent influence ils function.

Objects and environments are better suited for some functions than others. Roundwheels are better suited than square wheels for rolling; therefore, round wheelsare said to better afford rolling. Stairs are better suited than fences for climbing;therefore, stairs are said to better afford climbing. This is not to say that squarewheels cannot be rolled or fences climbed, rather that the physical characteristicsof round wheels and stairs better afford the functions of rolling and climbing.'

When the affordance of an object or environment corresponds with its intendedfunction, the design will perform more efficiently and will be easier to use.Conversely, when the affordance of an object or environment conflicts with itsintended function, the design will perform less efficiently and be more difficult touse. For example, a door with a handle affords pull ing. Someiimes, doors withhandles are designed to open only by pushing-the affordance of the handleconflicts with the door's function. Replace the handle with a flat plate, and it nowaffords pushing-the affordance of the flat plate corresponds to the way in whichthe door can be used. The design is improved.

lmages of common physical objects and environments can enhance the usabil ityof a design. For example, a drawing of a three-dimensional button on a computerscreen leverages our knowledge of the physical characteristics of buttons and,therefore, appears to afford pressing. The popular "desktop" metaphor used bycomputer operating systems is based on this idea-images of common items liketrash,cans and folders leverage our knowledge of how those items function in thereal world and, thus, suggest their function in the software environment.'

Whenever possible, you should design objects and environments to afford theirintended function, and negatively afford improper use. For example, stackablechairs should only stack one way. Mimic familiar objects and environments inabstract contexts (e.g., software interfaces) to imply the way in which new systemscan be used. When affordances are successfully employed in a design, it willseem inconceivable that the desisn could function or be used othenryise.

See alsc Constraint, Desire Line, Mapping, and Nudge.

I The seminal work on affordances is "TheTheory of Affordances" by James Gibson,in Perceiving, Acting, and Knowingby R.t. Shaw & J. Bransford (Eds), LawrenceErlbaum Associates, 1977; and The EcologicalApproach to Visual Perception by JamesGibson, Houghton Mifflin, 1979. A populartreatment of affordances can be found in lheDesign of Everyday Thingsby Donald Norman,Doubleday, 1990.

'? Note that the Ierm affordance refers to thepropefties of a physical object or environmento1ly. When images of physical objects orervironments are used (e.g.. image o[ abutton), the images, themselves, do not affordanything. The knowledge of button affordancesexists in the mind of the perceiver basedol experjence wilh physical butlons it isnot a property of the image. Therefore, theatfordance is said to be perceived. See, forexample, "Affordances and Desrgn" by DonaldNorman, wrwv..1nd.org.

22 Universal Pr inci i r les c l Design

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