excerpt from my Master's thesis
One of the most interesting concepts arising from my research and development of tangible interfaces is the idea of external legibility. While the HCI literature is full of examples of studies of interface legibility or how well an individual user or a group of users can interact with or understand an interface or interaction techniques that they are directly involved with using (what could be called internal legibility), there are hardly any examples of studies to examine the impact of interface design on non-participating observers. I define this property of interface design as external legibility.
External Legibility: a property of user interfaces that affects the ability of non-participating observers to understand the context of a user’s actions.
One reason why external legibility is important in interface design has to do with its relationship to semantics. Although it may never be possible to truly understand another’s mind, communication is based on shared understanding. Without a context in which to base understanding, inferring meaning or semantics becomes difficult.
Think of watching a master craftsperson working on a cabinet. You can see her hammering a nail to join two two-by-fours, you can see how she makes precise cuts along the edge of a piece of plywood. The context that the craftsperson works within is highly legible to an observer—the feeling of the wood, the knowledge of why a hammer is used, the memory of experiences of doing things like what the craftsperson are doing are available to many of us, but unless you too are a master craftsperson you may not know why she is doing the things that she does. The specific content of her actions are private, her thoughts and strategies, but the context of her actions are public. Without the ability to move from observation to inference accurately, it is hard to create shared understanding. External legibility is a measure of the reliability of the connection between observation and inference in interface design, but not in the traditional framing of one person and one machine—what could be called legibility. External legibility is a property of the space between one person observing another person using a machine.