 Systems with an easy to use or "English like" syntax are, however, quite distinct from systems that use a rich lexicon and include an internal representation (often as first order logic) of the semantics of natural language sentences.Hence the breadth and depth of "understanding" aimed at by a system determine both the complexity of the system (and the implied challenges) and the types of applications it can deal with.The "breadth" of a system is measured by the sizes of its vocabulary and grammar.