8 
 
3.1.2 Properties 
Properties are binary relations between individuals. I.e., properties link two individuals together. For 
example, the property hasFriend might link the individual Biswanath to the individual Michael, or 
the property hasChild might link the individual Michael to the individual Oriana. Properties can have 
inverses. For example, the inverse of hasChild is hasParent. Properties can be limited to having a 
single value – i.e., to being functional. They can also be transitive or symmetric. These property 
characteristics are explained in detail in Section 4.8. Figure 3.2 shows a representation of some properties. 
 
 
 
 
3.1.3 Classes 
OWL classes are sets that contain individuals. They are described using formal (mathematical) 
descriptions that rigorously define the requirements for membership of the class. For example, the class 
Cat would contain all the individuals that are cats in our domain of interest.2 Classes may be organized 
into a superclass-subclass hierarchy, which is also known as a taxonomy. However, taxonomies are often 
trees. I.e., each node has only one parent node. Class hierarchies in OWL are not restricted to be trees and 
multiple inheritance can be a powerful tool to represent data in an intuitive manner.  
Subclasses specialize (aka are subsumed by) their superclasses. For example, consider the classes Animal 
and Dog – Dog might be a subclass of Animal (so Animal is the superclass of Dog). This says that All 
dogs are animals, All members of the class Dog are members of the class Animal. OWL and Protégé 
 
2 Individuals can belong to more than one class and classes can have more than one superclass. Unlike OOP where 
multiple inheritance is typically unavailable or discouraged it is common in OWL.  
Properties are similar to properties in Object-Oriented Programming (OOP).  However, there are 
important differences between properties in OWL and OOP.   The most important difference is that OWL 
properties are first class entities that exist independent of classes. OOP developers are encouraged to 
read: https://www.w3.org/2001/sw/BestPractices/SE/ODSD/  
Michael 
Oriana 
hasChild 
Person 
 Country 
USA 
Italy 
India 
livesIn 
Buddy 
Rover 
Dog 
hasPet 
Figure 3.3: Representation of Classes containing Individuals