Affordances and Constraints

Note-taking for User Experience Design with June Ahn

Don Norman discusses affordances and constraints in The Design of Everyday Things, Chapter Four: Knowing What To Do.

Don Norman - The Design of Everyday Things

User experience design is easy in situations where there’s only one thing that the user can possibly do. But as the possibilities multiply, so do the challenges. We can deal with new things using information from our prior experiences, or by being instructed. The best-designed things include the instructions for their own use, like video games whose first level act as tutorials, or doors with handles that communicate how you should operate them by their shape and placement.

We use affordances and constraints to learn how things work. Affordances suggest the range of possibilities, and constraints limit the alternatives. Constraints include:

  • Physical limitations. Door keys can only be inserted into keyholes vertically, but you can still insert the key upside down. Car keys work in both orientations.
  • Semantic constraints. We know that red lights mean stop and green lights mean go, so we infer that a red light means a device is off or inoperative, and a green light means it’s on or ready to function. We have a slow cooker that uses lights in the opposite way and it screws me up every time.
  • Cultural constraints. Otherwise known as conventions. (Not sure how these are different from semantic constraints.) Somehow we all know without being told that we’re supposed to face forward in the elevator. Google Glass was an epic failure because its early adopters ran into the cultural constraint of people not liking to be photographed without consent.
  • Logical constraints. The arrangement of knobs controlling your stove burners should match the arrangement of the burners themselves.

The absence of constraints makes things confusing. Norman gives examples of how much designers love rows of identical switches which give no clues as to their function. Distinguishing the switches by shape, size, or grouping might not look as elegant, but would make it easier to remember which one does what thing.

Helpful designs use visibility (making the relevant parts visible) and feedback (giving actions an immediate and obvious effect.) Everyone hates the power buttons on iMacs because they’re hidden on the back, flush with the case. Feedback is an important way to help us distinguish the functional parts from the decorative ones. Propellerheads Reason is an annoying program because its skeuomorphic design puts as many decorative elements on the screen as functional ones. Ableton Live is easier to use because everything on the screen is functional.

When you can’t make things visible, you can give feedback via sound. Pressing a Mac’s power button doesn’t immediately cause the screen to light up, but that’s okay, because it plays the famous startup sound. Norman’s examples of low-tech sound feedback include the “zzz” sound of a functioning zipper, a tea kettle’s whistle, and the various sounds that machines make when they have mechanical problems. The problem with sound as feedback is that it can be intrusive and annoying.

The term “affordance” is the source for a lot of confusion. Norman tries to clarify it in his article “Affordance, Conventions and Design.” He makes a distinction between real and perceived affordances. Anything that appears on a computer screen is a perceived affordance. The real affordances of a computer are its physical components: the screen itself, the keyboard, the trackpad. The MusEDLab was motivated to create the aQWERTYon by considering the computer’s real affordances for music making. Most software design ignores the real affordances and only considers the perceived ones.

Designers of graphical user interfaces rely entirely on conceptual models and cultural conventions. (Consider how many programs use a graphic of a floppy disk as a Save icon, and now compare to the last time you saw an actual floppy disk.) For Norman, graphics are perceived affordances by definition.

Joanna McGrenere and Wayne Ho try to nail the concept down harder in “Affordances: Clarifying and Evolving a Concept.” The term was coined by the perceptual psychologist James J. Gibson in his book The Ecological Approach to Visual Perception. For Gibson, affordances exist independent of the actor’s ability to perceive them, and don’t depend on the actor’s experiences and culture. For Norman, affordances can include both perceived and actual properties, which to me makes more sense. If you can’t figure out that an affordance exists, then what does it matter if it exists or not?

Norman collapses two distinct aspects of design: an object’s utility of an object and the way that users learn or discover that utility. But are designing affordances and designing the information about the affordances the same thing? McGrenere and Ho say no, that it’s the difference between usefulness versus usability. They complain that the HCI community has focused on usability at the expense of usefulness. Norman says that a scrollbar is a learned convention, not a real affordance. McGrenere and Ho disagree, because the scrollbar affords scrolling in a way that’s built into the software, making it every bit as much a real affordance as if it were a physical thing. The learned convention is the visual representation of the scrollbar, not the basic fact of it.

The best reason to distinguish affordances from their communication or representation is that sometimes the communication gets in the way of the affordance itself. For example, novice software users need graphical user interfaces, while advanced users prefer text commands and keyboard shortcuts. A beginner needs to see all the available commands, while a pro prefers to keep the screen free of unnecessary clutter. Ableton Live is a notoriously beginner-unfriendly program because it prioritizes visual economy and minimalism over user handholding. A number of basic functions are either invisible or so tiny as to be effectively invisible. Apple’s GarageBand welcomes beginners with photorealistic depictions of everything, but its lack of keyboard shortcuts makes it feel like wearing oven mitts for expert users. For McGrenere and Ho, the same feature of one of these programs can be an affordance or anti-affordance depending on the user.

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Ethan Hein

Ethan Hein teaches music technology and music education at NYU and Montclair State University. He maintains an influential and widely-followed music blog at and has also recently written for NewMusicBox, Quartz, and Slate. He is an active producer and composer, and you can listen to his recent work here: Recently, musicians in eight countries created twenty recordings of his laptop orchestra composition “Divergence/Convergence” as part of a project by the Disquiet Junto, an online electronic music collective. As a founding member of the NYU Music Experience Design Lab, Ethan designs and researches new interfaces for music learning and expression.

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