What are the Ontology Formats?

RDF is the underlying data model, but there are several serialization formats for writing it down. Each has trade-offs in readability, verbosity, and tool support.

The RDF Data Model

The Resource Description Framework (RDF) represents knowledge as a set of triples: subject – predicate – object. Subjects and predicates are IRIs; objects can be IRIs or literal values (strings, numbers, dates). A collection of triples forms a directed graph.

The format (or serialization) determines how those triples are written in a file. The same graph can be serialized in Turtle, RDF/XML, JSON-LD, or any other RDF format — the information content is identical.

Turtle

Turtle (Terse RDF Triple Language) is the most popular human-readable RDF format. It supports prefix abbreviations, multi-value shorthand with commas, and multi-predicate shorthand with semicolons. OntoKit uses Turtle as its canonical format.

@prefix ex:   <http://example.org/> .
@prefix rdfs: <http://www.w3.org/2000/01/rdf-schema#> .
@prefix owl:  <http://www.w3.org/2002/07/owl#> .

ex:Animal a owl:Class ;
    rdfs:label "Animal"@en ;
    rdfs:comment "A living organism that feeds on organic matter."@en .

ex:Dog a owl:Class ;
    rdfs:subClassOf ex:Animal ;
    rdfs:label "Dog"@en .

RDF/XML

The original W3C serialization from 1999. Verbose but widely supported by XML toolchains. Harder for humans to read but excellent for machine-to-machine exchange.

<rdf:RDF xmlns:rdf="http://www.w3.org/1999/02/22-rdf-syntax-ns#"
         xmlns:rdfs="http://www.w3.org/2000/01/rdf-schema#"
         xmlns:owl="http://www.w3.org/2002/07/owl#">
  <owl:Class rdf:about="http://example.org/Animal">
    <rdfs:label xml:lang="en">Animal</rdfs:label>
  </owl:Class>
  <owl:Class rdf:about="http://example.org/Dog">
    <rdfs:subClassOf rdf:resource="http://example.org/Animal"/>
    <rdfs:label xml:lang="en">Dog</rdfs:label>
  </owl:Class>
</rdf:RDF>

N-Triples

A line-based format where each line is exactly one triple with full IRIs (no prefixes). Extremely simple to parse and ideal for streaming or bulk loading, but very verbose.

<http://example.org/Animal> <http://www.w3.org/1999/02/22-rdf-syntax-ns#type> <http://www.w3.org/2002/07/owl#Class> .
<http://example.org/Animal> <http://www.w3.org/2000/01/rdf-schema#label> "Animal"@en .
<http://example.org/Dog> <http://www.w3.org/1999/02/22-rdf-syntax-ns#type> <http://www.w3.org/2002/07/owl#Class> .
<http://example.org/Dog> <http://www.w3.org/2000/01/rdf-schema#subClassOf> <http://example.org/Animal> .
<http://example.org/Dog> <http://www.w3.org/2000/01/rdf-schema#label> "Dog"@en .

JSON-LD

JSON-LD embeds RDF data in standard JSON using a @context block. Popular with web developers because it integrates directly with JavaScript and REST APIs. Used by Schema.org for structured data in web pages.

{
  "@context": {
    "ex": "http://example.org/",
    "rdfs": "http://www.w3.org/2000/01/rdf-schema#",
    "owl": "http://www.w3.org/2002/07/owl#"
  },
  "@graph": [
    {
      "@id": "ex:Animal",
      "@type": "owl:Class",
      "rdfs:label": { "@value": "Animal", "@language": "en" }
    },
    {
      "@id": "ex:Dog",
      "@type": "owl:Class",
      "rdfs:subClassOf": { "@id": "ex:Animal" },
      "rdfs:label": { "@value": "Dog", "@language": "en" }
    }
  ]
}

Notation3 (N3)

N3 is a superset of Turtle that adds formulas, variables, and built-in predicates for expressing rules. While less common for publishing ontologies, it is used in reasoning and logic programming contexts.

Format Comparison

Format	Readability	Verbosity	Best For
Turtle	High	Low	Human authoring, version control
RDF/XML	Low	High	Legacy tools, XML pipelines
N-Triples	Low	Very high	Bulk loading, streaming
JSON-LD	Medium	Medium	Web APIs, JavaScript apps
N3	High	Low	Rules, logic programming