Modularity is a key requirement for large ontologies in order to achieve re-use, maintainability, and evolution. Mechanisms for 'normalisation' to achieve analogous aims are standard for databases. However, no similar notion of normalisation has yet emerged for ontologies. This paper proposes initial criteria for a two-step normalisation of ontologies implemented using OWL or related DL based formalisms. For the first - "ontological normalisation" - we accept Welty and Guarino's analysis. For the second - "implementation normalisation" - we propose an approach based on decomposing ("untangling") the ontology into independent disjoint skeleton taxonomies restricted to be simple trees, which can then be recombined using definitions and axioms to represent the relationships between them explicitly.
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