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A modularity concept for structuring large software systems is presented. The concept enforces an extreme modularity discipline that goes considerably beyond the one found in modern programming languages such as MODULA-2 or Ada. The concept is meant to be used to tightly control side effects in the execution of systems that are constructed of independently developed modules. A family of specification languages is introduced whose members are all based on the modularity concept and thus support the uniform monolinguistic specification of software systems at all development stages. The languages have been defined to enable matching informal, semiformal, and formal specifications and thus to make formal specification of modular systems practicable. The construction of large software systems as interconnections of modules is shown to lead to manageable system structures and to new degrees of freedom in the structuring of the software development process. The suitability of the modularity concept has been evaluated in a large software project for the development of a database management system. The concept and specification languages are explained with the aid of sample specifications.