The Design of Everyday Things

The ones we are concerned with here are simpler: they reside in the minds of the people who are using the product, so they are also “mental models.” Mental models, as the name implies, are the conceptual models in people’s minds that represent their understanding of how things work. Different people may hold different mental models of the same item

Affordances determine what actions are possible. Signifiers communicate where the action should take place. We need both.