Simply put, a fuzzy number is any fuzzy subset of the set of real numbers. Fuzzy numbers are quite useful in the mathematical formulation of vague statements about real numbers. Thus, the fuzzy subset “about 2” that was discussed in Sect.  3.13 is a fuzzy number. There are fuzzy systems where some of their variables range over states that are fuzzy numbers. Typically, these fuzzy numbers may represent terms such as “quite big,” “very big,” “somewhat small,” etc. If this is the case, then these variables are called linguistic variables.