So many user interfaces have the appearance of a collection of labels, stuck onto invisible boxes whose contents remain a mystery to users until they have made the effort of opening up each box in turn and sifting through its contents. In order to explore what might be called “the language of labeling,” we must first make some observations about the relationship between terms and concepts. Terms are words with special subject meanings; a term may consist of one or more “units” (e.g., user interface). As has been pointed out by Sager (1990), concepts are notoriously difficult to define; it is, however, possible to group them into four basic types: • class concepts or entities, generally corresponding to nouns • property concepts or qualities, for the most part corresponding to adjectives • relation concepts realized though various parts of speech, such as prepositions • function concepts or activities, corresponding to nouns and verbs Looking at the relationship between terms and concepts will help us to think about whether terms can be used to label various types of knowledge and also whether they can properly represent users’ knowledge needs. The present book is structured around linguistic “concepts” in the broad sense, whereas in this chapter, when we refer to concepts, it is in the narrower terminological sense indicated above. “We can use any names we wish as labels for concepts so long as we use them consistently. The only other criterion is convenience” In special subject areas, these same criteria apply, except that communication of specialized knowledge obliges us to take account of how concepts have been labeled by others and how the concepts we are handling fit into a wider scheme. We can draw up systems of concepts and try to specify relationships between them, uncovering along the way the knowledge structures that bind them together. However, we cannot do the same with terms. Terms are existential in nature, that is to say, they signal the existence of an entity, a relationship, an activity, or a quality.