We already mentioned that one of the benefits of TDD is executable documentation that is always up to date. However, documentation obtained through unit tests is often not enough. When working at such a low level, we get insights into details; however, it is all too easy to miss the big picture. If, for example, you were to inspect specifications that we created for the Tic-Tac-Toe game, you might easily miss the point of the application. You would understand what each unit does and how it interoperates with other units, but would have a hard time grasping the idea behind it. To be precise, you would understand that unit X does Y and communicates with Z; however, the functional documentation and the idea behind it would be, at best, hard to find.

The same can be said for development. Before we start working on specifications in the form of unit tests, we need to get a bigger picture. Throughout this book, you were presented with requirements that we used for writing...