In the language Ada, the set of values of a variable that are guaranteed to be exactly represented for the requested accuracy of the variable. The implementation will typically use a greater accuracy which has a larger range than that requested. Consequently the implemented accuracy will usually be higher, and other values will also be exactly represented. Associated with each implemented value will be a model interval, which defines the degree of uncertainty in the value. If a value is a model number then the model interval is zero; if a value is not a model number then the model interval is defined by the two model numbers surrounding the value.