Levels of abstraction in data base system architecture are investigated. A diagrammatic representation of abstract machines in a data base system is presented. Two major levels in a coexistence architecture: the data base modelling and data base model support levels, are considered in detail. A comprehensive treatment of abstractions in data base modelling is made and a new model, DATAM, is presented. Also presented is PRIMDAS, a primitive data structure interface for generalized model support. Applications of PRIMDAS and DATAM are given, and details of an actual implementation are described.