The relational model for data base organization introduced clearly defined basic algebraic concepts whose properties are well understood. As a consequence, questions of a semantic nature arise. Due to the mathematical nature of the relational model, these questions cannot be answered completely by it. Therefore, a data model is discussed that takes into account basic semantic properties that object types and relationships among them apparently have in the real world. The model permits a data base to be partitioned into independent object types which are natural insertion and deletion units, and associations among them. Independent object types are described by characteristics which are meaningful modification units.This data model permits us to analyze the ideas behind the relational theory more precisely, in particular the meaning of functional dependency and normalization. Insertion-deletion rules for relations in third normal form are developed, and proposals are made to avoid some drawbacks that exist even when relations in third normal form are used as the user interface to a data base.