Software and its code works like a clockwork. Small parts gather to a bigger entity operating one or more tasks, several of those entities gather to work together and produce the result. All those single commands and conditions form the foundation of software. Unlike a clockwork, the source code should not be in a black box nor should it be a compact and concentrated. Its essential parts and their function must be easy to identify. In order to be able to modify, exchange or extend all those parts, separation of concern is the key to success. Abstraction layer, packaging and modularity are all method to provide separation of concern.