Classes, Properties, and Triples#

Like any language, an ontology has different types of “words” that we use to express meaning. Let’s break these down:

Classes and Individuals#

Classes are like categories or types of things. Think of them as nouns in a sentence:

  • A Person (schema:Person)

  • A Book (schema:Book)

  • A Material (emmo:Material)

Individuals are specific instances of these classes. If “Person” is the class, then “Marie Curie” would be an individual belonging to that class. We express this relationship like this:

ex:MarieCurie rdf:type schema:Person .

Properties#

Properties are like verbs - they describe relationships between things or their characteristics. For example:

  • hasAuthor (connects a book to its author)

  • birthDate (specifies when someone was born)

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)

Building Statements with Triples#

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"

Types of Properties#

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

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