The mechanism of inheritance is a powerful tool used to describe complexity of a reality fraction. It is particularly important for knowledge representation systems modelling. It provides a specific ability to take over properties from a base element, what is crucial for conciseness, and modelling efficiency as well. In the Association-Oriented Database Metamodel, inheritance retains coherence with the object-oriented model in the most general terms. However, it has been otherwise defined which stems from the specificity of the metamodel, and particularly from its capabilities which have blazed a trail for its further extension. The main contribution of this article is a description of preliminary assumptions, postulates and conceptual solutions applicable to inheritance. They have been discussed against the background of the Association-Oriented Database Metamodel as well as an objectoriented model compared with the former.