Legibility: Why Must a Correct State Also Be Easy to Read?

Legibility is a system’s capacity to present the state, boundary and consequence relevant to action in a form that is easy to recognise. An elevator’s current floor, a file’s failed synchronisation or the people covered by a rule may all be internally determined without being clear to the user. The problem is not that the system does not know; it is that a person cannot act reliably on what is shown.

Legibility does not mean providing more information. A screen displaying twenty parameters may be harder to judge than one unambiguous red light. The task is to choose the right level: make the distinctions needed for the present decision visible and leave irrelevant complexity behind it. Legibility also differs from observability. Observability asks whether hidden internal states can be inferred from signals; legibility asks whether the relevant state has already been organised into a humanly understandable form.

Legibility joins transparency to usability. Information may be available yet still fail to support judgement because names are inconsistent, hierarchy is unclear or feedback arrives too late. Good legibility does not make the world simple. It gives a complex world a clear point of entry for a particular action.