Evolving out of theoretical and practical work, the paper presents the motivation and basic ideas for the construction and use of modular knowledge bases. The approach relates to earlier work carried out by each of the two authors of the paper separately. A model is introduced that merges the two previous approaches, modules for logical knowledge bases, and ordering by generality domains, while maintaining their benefits. Central aims are reusability, the restriction of memory searching, and the management of inconsistent (competing) knowledge within one knowledge base. The model is explained using examples, and the formal semantics are discussed of structured, modular knowledge bases for knowledge representations that are based on logic programming.