Each class and relation (property) in the ontology must have a unique URI identifier. The URI should be constructed from a base URI, a prefix that is unique within the Foundry (e.g. GO, CHEBI, CL) and a local identifier (e.g. 0000001). The local identifier should not consist of labels or mnemonics meaningful to humans. Additional information is available at

The ID-space / prefix must be registered with the OBO Library in advance. Please see the relevant documentation.

This check is automatically validated.

OBO-Format Ontologies

If the source ontology is in OBO-Format, then this is automatically satisfied so long as all IDs are of the form <IDSPACE> : <NUMBER>

Date Accepted

  • original principle


Original Formulation

 The ontologies possesses a unique identifier space within
the OBO Foundry.

The source of a term (i.e. class) from any ontology can be immediately
identified by the prefix of the identifier of each term. It is,
therefore, important that this prefix be unique. 


The OBI class ‘imaging assay’ has the following URI:


  • There are systems that use alphanumeric id’s. This should be discouraged, especially as these have semantic content.