Fuzzy logic has two different meanings. In a narrow sense, fuzzy logic is a logical system, which is an extension of multivalued logic. However, in a wider sense fuzzy logic is almost synonymous with the theory of fuzzy sets, a theory which relates to classes of objects with unsharp boundaries in which membership is a matter of degree. Another basic concept in fuzzy logic, which plays a central role in most of its applications, is that of a fuzzy if-then rule or, simply, fuzzy rule. [