This paper develops a fundamental understanding of multidatabase concurrency control, which reveals the principles of multidatabase concurrency control to accommodate the two-level hierarchy of globally and locally independent transaction mallagemcnts. Local extensibility is proposed as a unifying principle in the combination of global and local transaction managements. Under this general principle, the key issues of ensuring the isolation and semantic atomicity of global transactions are addressed. Correctness criteria are formulated for various situations and recoverable global transactions arc constructed. Our discussions are conduded upon the only basic assumptions of serializability and recoverability all local database systems. The preservation of the necessary balance between the demands of global transaction management and local autonomy provide us with an enhanced theoretical understanding of the limitations of global transaction management in multidataba.se systems.