The document describes DOOR (Descriptive Ontology of Ontology Relations), which characterizes relations between ontologies. It identifies relevant ontology relations from OWL, RDFS and SKOS. It then distinguishes sub-relations by analyzing ontologies from five perspectives. Each relation is characterized by algebraic properties and a taxonomic structure is established between relations. Rules are introduced to define complex relations combining simpler ones. The methodology is repeated for other relations to build the DOOR ontology of ontology relations.