Owl

OWL & Ontology Design Skill

Design, implement, and maintain ontologies with an ontologist's perspective. This skill embodies the practical wisdom that ontology is fundamentally about modeling the meaningful relationships between things—and recognizing that the field is evolving from class-centric OWL toward shape-centric SHACL.

Core Philosophy (Cagle's Principles)

"OWL is not going away, but there is definitely a shift occurring within the semantic community around the use of SHAPES, rather than classes and properties."

Key Insights:

1. RDFS does more than you think: "It is possible to do a surprising amount with even the minimal set of RDFS"
2. OWL brought inference power: "Most of the real power came from the introduction of OWL, which established inferential predicates that trigger rules generating additional triples"
3. Reasoners are fading: "Reasoners are disappearing from newer knowledge graph systems, partly because SPARQL and SPARQL Update are more targeted"
4. The future is shapes: "SHACL represents a shift by the W3C away from OWL2 and towards a workflow that takes advantage of SPARQL"
5. Learn all three: "Learn SHACL, RDFS and OWL. These are the languages that underlie almost the entire RDF stack"

Guide Router

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The Ontology Landscape

Taxonomy of Ontologies (Cagle's Classification)

The RDFS → OWL → SHACL Evolution

RDFS (2000)          OWL (2004/2009)         SHACL (2017)
    │                      │                      │
    │ Basic typing         │ Rich inference       │ Flexible validation
    │ rdfs:Class           │ owl:Class            │ sh:NodeShape
    │ rdfs:subClassOf      │ owl:equivalentClass  │ sh:targetClass
    │ rdfs:domain/range    │ owl:Restriction      │ sh:property
    │                      │ Reasoners required   │ SPARQL-based
    │                      │                      │
    └──────────────────────┴──────────────────────┘
                     Knowledge Graphs Today

RDFS Quick Reference

RDF Schema provides the foundational vocabulary for typing and hierarchy.

Core Classes

@prefix rdfs: <http://www.w3.org/2000/01/rdf-schema#> .
@prefix rdf: <http://www.w3.org/1999/02/22-rdf-syntax-ns#> .

# rdfs:Resource - Everything
# rdfs:Class - The class of classes
# rdfs:Literal - String/numeric values
# rdf:Property - The class of properties
# rdfs:Datatype - Typed literal classes

Core Properties

RDFS Example

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

ex:Person a rdfs:Class ;
    rdfs:label "Person" ;
    rdfs:comment "A human being" .

ex:Employee a rdfs:Class ;
    rdfs:subClassOf ex:Person ;
    rdfs:label "Employee" .

ex:worksFor a rdf:Property ;
    rdfs:domain ex:Employee ;
    rdfs:range ex:Organization ;
    rdfs:label "works for" .

RDFS Inference Rules

OWL 2 Quick Reference

OWL extends RDFS with rich class expressions and property characteristics.

Built-in Classes

owl:Thing        # Universal class (everything)
owl:Nothing      # Empty class (nothing)

Class Axioms

# Subclass
ex:Student rdfs:subClassOf ex:Person .

# Equivalence (bidirectional subclass)
ex:Human owl:equivalentClass ex:Person .

# Disjointness (no shared members)
ex:Cat owl:disjointWith ex:Dog .

# Disjoint Union (partitions a class)
ex:Animal owl:disjointUnionOf ( ex:Mammal ex:Bird ex:Fish ex:Reptile ) .

Class Expressions (Turtle)

@prefix owl: <http://www.w3.org/2002/07/owl#> .

# Intersection (AND)
ex:WorkingParent owl:equivalentClass [
    owl:intersectionOf ( ex:Person
                         [ a owl:Restriction ;
                           owl:onProperty ex:hasChild ;
                           owl:someValuesFrom ex:Person ]
                         [ a owl:Restriction ;
                           owl:onProperty ex:hasJob ;
                           owl:someValuesFrom ex:Job ] )
] .

# Union (OR)
ex:Parent owl:equivalentClass [
    owl:unionOf ( ex:Mother ex:Father )
] .

# Complement (NOT)
ex:NonParent owl:equivalentClass [
    owl:complementOf ex:Parent
] .

# Enumeration (one of)
ex:PrimaryColor owl:equivalentClass [
    owl:oneOf ( ex:Red ex:Blue ex:Yellow )
] .

Property Restrictions

# Existential (some)
ex:Parent owl:equivalentClass [
    a owl:Restriction ;
    owl:onProperty ex:hasChild ;
    owl:someValuesFrom ex:Person
] .

# Universal (only/all)
ex:HappyParent rdfs:subClassOf [
    a owl:Restriction ;
    owl:onProperty ex:hasChild ;
    owl:allValuesFrom ex:HappyPerson
] .

# Exact cardinality
ex:Couple rdfs:subClassOf [
    a owl:Restriction ;
    owl:onProperty ex:hasMember ;
    owl:cardinality "2"^^xsd:nonNegativeInteger
] .

# Min/Max cardinality
ex:LargeFamily rdfs:subClassOf [
    a owl:Restriction ;
    owl:onProperty ex:hasChild ;
    owl:minCardinality "3"^^xsd:nonNegativeInteger
] .

# Has value
ex:BostonResident rdfs:subClassOf [
    a owl:Restriction ;
    owl:onProperty ex:livesIn ;
    owl:hasValue ex:Boston
] .

# Self restriction
ex:Narcissist owl:equivalentClass [
    a owl:Restriction ;
    owl:onProperty ex:loves ;
    owl:hasSelf true
] .

Qualified Cardinality (OWL 2)

# At least 2 children who are students
ex:ProudParent rdfs:subClassOf [
    a owl:Restriction ;
    owl:onProperty ex:hasChild ;
    owl:minQualifiedCardinality "2"^^xsd:nonNegativeInteger ;
    owl:onClass ex:Student
] .

Property Characteristics

Object Property Features

# Functional (at most one value)
ex:hasBirthMother a owl:FunctionalProperty .

# Inverse Functional (at most one subject per value)
ex:hasSocialSecurityNumber a owl:InverseFunctionalProperty .

# Symmetric (bidirectional)
ex:isMarriedTo a owl:SymmetricProperty .

# Asymmetric (never bidirectional)
ex:isParentOf a owl:AsymmetricProperty .

# Transitive (chains)
ex:isAncestorOf a owl:TransitiveProperty .

# Reflexive (everything relates to itself)
ex:knows a owl:ReflexiveProperty .

# Irreflexive (nothing relates to itself)
ex:isParentOf a owl:IrreflexiveProperty .

Property Relationships

# Inverse properties
ex:hasParent owl:inverseOf ex:hasChild .

# Equivalent properties
ex:creator owl:equivalentProperty dc:creator .

# Subproperty
ex:hasMother rdfs:subPropertyOf ex:hasParent .

# Disjoint properties (OWL 2)
ex:likes owl:propertyDisjointWith ex:hates .

Property Chains (OWL 2)

# uncle = parent's brother
ex:hasUncle owl:propertyChainAxiom ( ex:hasParent ex:hasBrother ) .

# grandparent = parent's parent
ex:hasGrandparent owl:propertyChainAxiom ( ex:hasParent ex:hasParent ) .

Keys (OWL 2)

# Social security number uniquely identifies a person
ex:Person owl:hasKey ( ex:hasSocialSecurityNumber ) .

# Composite key
ex:Course owl:hasKey ( ex:hasDepartment ex:hasCourseNumber ) .

OWL 2 Profiles

Choose the right profile based on your needs:

Profile Selection Guide

Do you need SQL query rewriting?
├── Yes → OWL 2 QL
└── No → Do you have millions of classes?
         ├── Yes → OWL 2 EL
         └── No → Do you need rule-based implementation?
                  ├── Yes → OWL 2 RL
                  └── No → OWL 2 DL

Profile Feature Comparison

SKOS Quick Reference

Simple Knowledge Organization System for taxonomies and controlled vocabularies.

Core Classes

@prefix skos: <http://www.w3.org/2004/02/skos/core#> .

skos:Concept           # A unit of thought
skos:ConceptScheme     # An aggregation of concepts
skos:Collection        # A group of concepts
skos:OrderedCollection # An ordered group

Labeling

ex:Cat a skos:Concept ;
    skos:prefLabel "Cat"@en, "Katze"@de ;
    skos:altLabel "Feline"@en, "Kitty"@en ;
    skos:hiddenLabel "Kat"@en .  # For search, misspellings

Semantic Relations

# Hierarchy (non-transitive by design)
ex:Mammal skos:broader ex:Animal .
ex:Cat skos:broader ex:Mammal .

# Transitive closure (for query expansion)
ex:Cat skos:broaderTransitive ex:Mammal, ex:Animal .

# Association (symmetric)
ex:Cat skos:related ex:Dog .

Documentation

ex:Cat a skos:Concept ;
    skos:definition "A small domesticated carnivorous mammal"@en ;
    skos:scopeNote "Use for domestic cats only"@en ;
    skos:example "Persian, Siamese, Tabby"@en ;
    skos:historyNote "Added in version 1.0"@en ;
    skos:editorialNote "Review needed"@en ;
    skos:changeNote "Broader term changed 2024-01"@en ;
    skos:notation "CAT-001" .

Concept Schemes

ex:AnimalTaxonomy a skos:ConceptScheme ;
    skos:prefLabel "Animal Classification"@en ;
    skos:hasTopConcept ex:Animal .

ex:Animal a skos:Concept ;
    skos:inScheme ex:AnimalTaxonomy ;
    skos:topConceptOf ex:AnimalTaxonomy .

Mapping Between Schemes

# High confidence - interchangeable
ex:Cat skos:exactMatch wikidata:Q146 .

# Similar but not identical
ex:Feline skos:closeMatch dbpedia:Cat .

# Cross-scheme hierarchy
ex:Cat skos:broadMatch ex:Pet .
ex:Siamese skos:narrowMatch ex:Cat .

Manchester Syntax Quick Reference

Human-readable OWL syntax used in Protégé and documentation.

Class Expressions

Class: Parent
    EquivalentTo: Person and hasChild some Person

Class: HappyParent
    SubClassOf: Parent and hasChild only HappyPerson

Class: Orphan
    EquivalentTo: Person and not (hasParent some Person)

Class: PrimaryColor
    EquivalentTo: {Red, Blue, Yellow}

Class: LargeFamily
    SubClassOf: hasChild min 3 Person

Property Definitions

ObjectProperty: hasParent
    Domain: Person
    Range: Person
    InverseOf: hasChild

ObjectProperty: hasAncestor
    SubPropertyOf: hasParent
    Characteristics: Transitive

ObjectProperty: hasSpouse
    Domain: Person
    Range: Person
    Characteristics: Symmetric, Functional

ObjectProperty: hasUncle
    SubPropertyChain: hasParent o hasBrother

Individual Assertions

Individual: John
    Types: Person, Parent
    Facts: hasChild Mary, hasChild Bob, hasAge 45

Individual: Mary
    Types: Student
    Facts: hasParent John
    SameAs: MarySmith
    DifferentFrom: MaryJones

The Shift to SHACL (Cagle's Perspective)

"SHACL represents a shift by the W3C away from OWL2 and towards a workflow that takes advantage of the power and expressiveness of SPARQL."

OWL vs SHACL Comparison

When to Use Each

Use OWL when:

• You need automated classification
• Inference is core to your use case
• You're building a formal ontology
• Interoperability with OWL tools matters

Use SHACL when:

• You need data validation
• You want graduated severity levels
• Your data may be incomplete
• You're integrating with SPARQL workflows
• You need closed-world validation

Migration Pattern

# OWL restriction
ex:Person rdfs:subClassOf [
    a owl:Restriction ;
    owl:onProperty ex:hasName ;
    owl:minCardinality 1
] .

# Equivalent SHACL shape
ex:PersonShape a sh:NodeShape ;
    sh:targetClass ex:Person ;
    sh:property [
        sh:path ex:hasName ;
        sh:minCount 1 ;
        sh:severity sh:Violation ;
        sh:message "Person must have at least one name"
    ] .

Common Ontology Patterns

Value Partition

Exhaustive, mutually exclusive categories:

# Values must be exactly one of these
ex:HealthStatus owl:equivalentClass [
    owl:unionOf ( ex:Healthy ex:Sick ex:Deceased )
] .

ex:Healthy owl:disjointWith ex:Sick, ex:Deceased .
ex:Sick owl:disjointWith ex:Deceased .

N-ary Relation

When a relationship needs attributes:

# Instead of: ex:John ex:bought ex:Book .
# Use an intermediate node:

ex:Purchase1 a ex:Purchase ;
    ex:buyer ex:John ;
    ex:item ex:Book ;
    ex:price "29.99"^^xsd:decimal ;
    ex:date "2025-01-15"^^xsd:date .

Class as Property Value

When a class itself is a value:

# Approach 1: Instance representing the class
ex:LionSpecies a ex:Species ;
    rdfs:label "Lion" ;
    ex:scientificName "Panthera leo" .

ex:Simba a ex:Animal ;
    ex:hasSpecies ex:LionSpecies .

# Approach 2: Punning (OWL 2)
ex:Lion a owl:Class, ex:Species .

Defined vs Primitive Classes

# Primitive: necessary conditions only
ex:Person a owl:Class .
ex:Person rdfs:subClassOf ex:Agent .

# Defined: necessary and sufficient
ex:Parent owl:equivalentClass [
    owl:intersectionOf (
        ex:Person
        [ a owl:Restriction ;
          owl:onProperty ex:hasChild ;
          owl:someValuesFrom ex:Person ]
    )
] .

Best Practices (Allemang & Hendler)

Modeling Principles

1. Model what you know: Don't over-specify
2. Use equivalentClass for definitions: Enables classification
3. Use subClassOf for constraints: Necessary conditions
4. Prefer object properties: Richer semantics than data properties
5. Avoid overloading classes: "One of the biggest problems that OWL has"

Common Anti-Patterns

Naming Conventions

Tools and Resources

Ontology Editors

• Protégé: Industry standard, supports OWL 2
• TopBraid Composer: Enterprise features, SHACL support
• WebVOWL: Visualization

Reasoners

• HermiT: OWL 2 DL, tableau-based
• Pellet/Stardog: OWL 2 DL, commercial
• ELK: OWL 2 EL, very fast
• RDFox: OWL 2 RL, rule-based

Resources

W3C Specifications

• OWL 2 Overview
• OWL 2 Primer
• OWL 2 Profiles
• RDF Schema 1.1
• SKOS Reference
• SHACL

Books

• Semantic Web for the Working Ontologist by Allemang, Hendler, Gandon
• Learning SPARQL by Bob DuCharme

Kurt Cagle's Work

• The Ontologist
• The Cagle Report