The schema of a database system consists of the constructs that model the entities of data. Schema evolution is the timely change of the schema and the consistent management of these changes. Dynamic schema evolution (DSE) is the management of schema changes while a database management system is in operation. DSE is a necessary facility of objectbase systems (OBSs) because of the volatile application domains that OBSs support. We propose a sound and complete axiomatic model for DSE in OBSs that supports the fundamental concepts of object-oriented computing such as subtyping and property inheritance. The model can infer all schema relationships from two identified input sets associated with each type called the essential supertypes and essential properties. These sets are typically specified by schema designers, but can be automatically supplied within an OBS. The inference mechanism performed by the model has a proven termination.