While the phase of designing a model is usually presented as quite distinct from the prototyping phase, the two are so integrally related that we really should discuss them together.

Like any other phase, these two have the dual purpose of validating the preceding steps and serving as the take-off point for the next phase. In fact, they represent the moment at which the specifications of the application really become visible. For that reason, the end-user is all the more crucial at this juncture in the development, especially if this application is new or one that is insufficiently specified.