As explained in the previous chapter about the [Semantic Web and Ontologies](/en/framework/semantic), NextGraph is based on RDF, and OWL is used to defined Ontologies, which are equivalent to a Schema definition.
Every document can list a set of prefixes and associated ontology that the Document is using.
This is done with the `context` branch internally and is accessible from the document Menu, under Tools, then Schema. (not implemented for now).
The schema is defined at the Document level, not at the branch or block level.
It can updated at any time, and prefixes can be removed too, that will not affect the predicates that are using it, as all predicates are encoded and saved with their full URI anyway.
The context is useful for SPARQL Queries and for JSON-LD output.
JSON-LD is not available yet but will be included in an upcoming release.
By default, NextGraph always defines the prefix `ng` that is reserved for special predicates that all Documents in Nextgraph can or must have.
The list is detailed below. This prefix cannot be overridden.
In this `ng` ontology, we also define the primary classes of all the types of documents that are officially supported in our Apps.
We also can expose some common prefixes if requested, that are listed a bit below.
And in general we strive to map our official primary classes with existing ontologies for backward compatibility. You are encouraged too to reuse existing ontologies or directly in your prefixes, or by inheriting from them in your own ontology definitions.
We will also keep a repository of well known ontologies of specific interest.
A new ontology can be defined by creating a new Document of type Data / Ontology (not available yet)
