A door with a flat metal plate at hand height doesn't need a sign — the plate itself says push. A door with a handle at hand height says, just as wordlessly, pull. When a door's actual hardware sends the wrong signal — a handle mounted on a door that only swings open one way — people get it wrong constantly, reaching for the handle and yanking, even with a printed "PUSH" sign taped right above it. Designers now call this a "Norman door," after Don Norman, whose 1988 book The Design of Everyday Things turned the everyday embarrassment of failing at a door into a foundational idea in design.
Affordance versus signifier
Norman drew a distinction that now runs through nearly all interface design. An affordance is what an object's actual physical properties allow you to do — a flat plate can be pushed; a handle can be pulled or gripped. A signifier is a perceptible cue that tells you which affordance to use — a shadow that makes something look raised, a shape, a label. A well-designed object's signifiers match its real affordances, so using it correctly feels obvious rather than something you have to be told. A badly designed one puts a pull-shaped signifier on a push-only affordance, and no amount of signage fully overrides the shape's own silent instruction.
Why flat design reopened an old argument
The distinction became newly relevant around 2013, when major software platforms — iOS 7, Windows — shifted from skeuomorphic interfaces, where buttons were styled to look raised and glossy, mimicking physical hardware, toward flat design: plain coloured rectangles with no shadow, gradient, or bevel. Critics argued flat design stripped out exactly the visual signifiers that used to make it obvious what was tappable, forcing users to rely on learned convention — "that word in blue is probably a link" — instead of a built-in visual cue doing the work automatically.
What we're still unsure about
The flat-design debate never fully resolved. Most major interfaces have quietly reintroduced subtle shadows, depth, and other signifiers since the starkest flat-design era — sometimes called "flat 2.0" — suggesting the purest version of the approach outran what users' learned conventions could actually cover on their own. How much usability depends on built-in perceptual signifiers, versus how much users will simply learn through repeated exposure to a convention, is still genuinely argued over among interface designers, not a settled trade-off with one correct answer.
This sits inside User Interface (UI) & User Experience (UX) Design, one of seven topics in Design, one of five domains in Art, one of seventeen subjects the app can quiz you on.