ontologies that follow OBO principles but are not officially part of OBO, such as eagle-i's Reagent Application Ontology. and the Animals in Context Ontology. An integration into OBO of the OntoClean's theory of rigidity has been proposed as a step to standardize candidate ontologies. This integration would make it easier to develop software to automatically check candidates. Tools The OBO Foundry community is also dedicated to developing tools to facilitate creating and maintaining ontologies. Most ontology developers in OBO use the Protégé ontology editor and the Web Ontology Language (OWL) for building ontologies. To facilitate command line management of ontologies in a Protégé- and OWL-compatible format, the OBO Foundry has developed the tool ROBOT (ROBOT is an OBO Tool). ROBOT aggregates functions for routine tasks in ontology development, is open source, and can be used either via the command line or as a library for any language on the Java Virtual Machine. Other tool related to the OBO effort is OBO-Edit, an ontology editor and reasoner funded by the Gene Ontology Consortium. There are also plugins for OBO-Edit which facilitate the development of ontologies, such as the semi-automatic ontology generator DOG4DAG. The OBO file format The OBO file format is a biology-oriented language for building ontologies. It is based on the principles of Web Ontology Language (OWL). As a community effort, standard common mappings have been created for lossless roundtrip transformations between Open Biomedical Ontologies (OBO) format and OWL. The research contains methodical examination of each of the constructs of OBO and a layer cake for OBO, similar to the Semantic Web stack. OBO Foundry Ontologies The initial set of OBO Foundry ontologies was composed by mature ontologies (such as the Gene Ontology, GO, and the Foundational Model of Anatomy, FMAO), by mergers of previously existing ontologies (ex: the Cell Ontology, CL, formed from different dedicated ontologies, and related parts on GO and FMAO) and by development of new ontologies based on its principles. The original set of ontologies also included the Zebrafish Anatomical Ontology (a part of the Zebrafish Information Network), the CheBI ontology, the Disease Ontology, the Plant Ontology, the Sequence Ontology, the Ontology for Biomedical Investigations and the Protein Ontology. The number of ontologies in OBO has grown to the order of hundreds, and they are gathered in the list of OBO Foundry ontologies. OBO Foundry and Wikidata A number of different OBO Foundry ontologies have also