In the preceding chapter, we saw that the design of a representation system requires choices on two different levels. First, there is the level of representation formalism, i.e., the choice of syntax and semantics for the representation. Then, there is the equally important choice of a representation language, i.e., the vocabulary to be made available in the chosen formalism. We also saw that this vocabulary is to be looked at not as a set of syntactical terms, but as a system of concepts.