We present the notion of {\\it consistency relation} in UML and B multi-view specifications. It is defined as a semantic relation between both views. It provides us with a sound basis to define the notion of development operator. An operator models a development step; it separates the design decisions from their expression in the specification formalisms. Operator correctness is defined as a property which guarantees that the application of an operator on a consistent specification state yields a consistent new state. An operator can be proven once and for all to be correct. A classical case-study, the Generalized Railroad Crossing (GRC), demonstrates how the different notions can be put in practice to provide specifiers with a realistic development model.