Classes, Properties, and Triples#

RDF statements are build from classes, individuals, and properties.

Classes and Individuals#

A class is a category of a thing. Think of them as nouns in a sentence, like "a person" (schema:Person) or "a book" (schema:Book).

An individual is a specific instance of a class. If "Person" is the class, then "Marie Curie" would be an individual belonging to that class. We can express this relationship as an RDF triple like this:

@prefix ex: <https://example.org/> .
@prefix rdf: <http://www.w3.org/1999/02/22-rdf-syntax-ns#> .
@prefix schema: <https://schema.org/> .

ex:MarieCurie rdf:type schema:Person .

Properties#

A Property connects resources or attaches values.

  • hasAuthor (connects a book to its author)

  • birthDate (specifies when someone was born)

RDF properties come in three main flavors:

  1. Object properties connect things to other things:

    ex:MyBook ex:hasAuthor ex:Alice .
# Connects one resource to another resource

  2. Data properties connect things to values:

    ex:Alice schema:birthDate "1965-07-20"^^xsd:date .
# Connects a resource to a specific value

  3. Annotation properties add human-readable information:

    schema:Book rdfs:comment "A written work, typically bound and published." .
# Adds documentation or metadata

Every property has rules about what it can connect:

  • The domain specifies what can appear at the start of the relationship

  • The range specifies what can appear at the end

For example, hasAuthor might have:

  • Domain: Creative works (only creative works can have authors)

  • Range: Persons (only persons can be authors)

Triples: the basic statement#

In RDF, we combine classes, individuals, and properties to make statements called triples. Each triple has three parts:

  • Subject (what we're talking about)

  • Predicate (the property or relationship)

  • Object (what we're saying about the subject)

For example:

# A class relationship
schema:Book rdf:type schema:CreativeWork .
# Means: "A Book is a type of Creative Work"

# An individual relationship
ex:MyBook ex:hasAuthor ex:Alice .
# Means: "MyBook was written by Alice"

Together, classes, properties, and triples form the foundation of an ontology, structuring knowledge in a way that is both human-readable and machine-processable.