Affordance is a visual clue to the function of an object.

The term, that was firstly brought out in the 70s by perceptual psychologist J. J. Gibson and originally meant “the actionable properties between the world and an actor (a person or animal)”, was introduced with the aforesaid significance by Don Norman‘s besteller The Psychology of Everyday Things (a.k.a POET) and immediately adopted by the design community.

Put in a down-to-earth way, we could say that Threadless scoring system’s affordance is very high since the big squared buttons function is easily and readily understood: the graphic elements stand out, clearly asserting their scope, as if they were screaming out  ”hit me!”.

A curiosity: Norman’s book was translated into Italian as The masochist’s percolator, which I know has almost nothing to do with the orginal title, but I think it’s ten thousand times funnier.