Layers are partitions of a system in which the members of each partition are aware of and are able to use the services of the layers “below,” but unaware of and independent of the layers “above.” When the dependencies of MODULES are drawn, they are often laid out so that a MODULE with dependents appears below its dependents. In this way, layers sometimes sort themselves out so that none of the objects in the lower levels is conceptually dependent on those in higher layers.