Royce's model describes a different waterfall model that can go back to previous phases. Often, it will become apparent at a particular phase that something went wrong in a previous phase (this will most commonly surface in testing phases). It should then be possible to go back to a previous phase easily. 99% of the time, changes to the design have to be made – nobody is perfect, so this model is more realistic. Nevertheless, Royce continued to emphasize the importance of good documentation for proper phase transitions.

The following diagram illustrates Royce's model: