### Defining Subclasses

The ability to define subclasses is a powerful feature of the directory. Structure rules govern which object classes can be children of which others in the DIT and therefore determine possible name forms.

The directory standard defines a number of standard attribute types and object classes. For example, the attribute types Common-Name and Description, and the object classes Country and Organizational-Person are defined. Implementations of the directory standard, such as DCE, define their own schemas following rules stated in the standard with additional attribute types and object classes.

The following figure shows the relationship between schemas and the directory information model.

The Relationship Between Schemas and the DIT

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