Figure 4

The ANSI-SPARC three layer database architecture was proposed in 1975, and is still used in modern RDBMS. The three proposed layers were a physical schema which defined how the data is actually stored (inode information), a conceptual schema which represented how information was related and indexed, and an external schema which represented how information was presented. The architecture was designed to provide immunity to change: the physical schema defined how the actual information was stored, and could be changed without effecting how external applications interacted with the data; and the external schema could be changed to define richer APIs, without having to change the underlying storage mechanism.