There are all sorts of ways a software system might be divided, but through experience and convention, the industry has converged on LAYERED ARCHITECTURES, and specifically a few fairly standard layers. The metaphor of layering is so widely used that it feels intuitive to most developers. Many good discussions of layering are available in the literature, sometimes in the format of a pattern (as in Buschmann et al. 1996, pp. 31–51). The essential principle is that any element of a layer depends only on other elements in the same layer or on elements of the layers “beneath” it.