If you think of the design of the three levels that we just talked about is like a house, the architecture level is like the foundation and overall structure of that house; once you've built it, you don't change it. The abstraction level is like the walls inside that house. Sometimes, you might add another wall to divide a big room into two smaller ones, or you might tear down a wall to create more space. But you do not need to do that often. The implementation level is like the furniture, carpets, wallpaper, curtains, and so on, which are all those small and movable items in a house. You can change these frequently, for example, you can move the sofa closer to the TV, replace the dining table with a new one, or do another round of redecoration. You also need to keep cleaning the house on a regular basis. Otherwise, it will turn into a mess, no matter how beautiful it was in its initial design.

In a software application, all of the details are in the source code, and that is also the part that requires your constant effort to keep it clean, simple, and expressive. Otherwise, you will see the devil in the details.