“Extreme Programming concentrates exclusively on the active elements of a program and executable tests. Even comments added to the code do not affect program behavior, so they always fall out of sync with the active code and its driving model. External documents and diagrams do not affect the behavior of the program, so they fall out of sync. On the other hand, spoken communication and ephemeral diagrams on whiteboards do not linger to create confusion. This dependence on the code as communication medium motivates developers to keep the code clean and transparent.”