21 
 
 
Figure 4.10 The New PizzaTopping Class Hierarchy 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
So far, we have created some simple named classes and subclasses which hopefully seem 
intuitive and obvious. However, what does it actually mean to be a subclass of something in 
OWL? For example, what does it mean for VegetableTopping to be a subclass of 
PizzaTopping? In OWL subclass means necessary implication. I.e., if VegetableTopping is a 
subclass of PizzaTopping then all instances of VegetableTopping are also instances of 
PizzaTopping. It is for this reason that we try to have standards such as having all 
PizzaTopping classes end with the word “Topping”. Otherwise, it might seem we are saying 
that anything that is a kind of Ham like the Ham in your sandwich is a kind of MeatTopping or 
PizzaTopping which is not what we mean.  For large ontologies strict attention to the naming 
of classes and other entities can prevent potential confusion and bugs.