logic individual. Classes A class is a collection of objects. A class may contain individuals, instances of the class. A class may have any number of instances. An instance may belong to none, one or more classes. A class may be a subclass of another, inheriting characteristics from its parent superclass. This corresponds to logical subsumption and DL concept inclusion notated . All classes are subclasses of owl:Thing (DL top notated ), the root class. All classes are subclassed by owl:Nothing (DL bottom notated ), the empty class. No instances are members of owl:Nothing. Modelers use owl:Thing and owl:Nothing to assert facts about all or no instances. Class and their members can be defined in OWL either by extension or by intension. An individual can be explicitly assigned a class by a Class assertion, for example we can add a statement Queen elizabeth is a(n instance of) human, or by a class expression with ClassExpression statements every instance of the human class who has a female value to the sex property is an instance of the woman class. Example Let's call human the class of all humans in the world is a subclass of owl:thing. The class of all women (say woman) in the world is a subclass of human. Then we have The membership of some individual to a class could be noted ClassAssertion( human George_Washington ) and class inclusion SubClassOf( woman human ) The first means "George Washington is a human" and the second "every woman is human". Properties A property is a characteristic of a class - a directed binary relation that specifies some attribute which is true for instances of that class. Properties sometimes act as data values, or links to other instances. Properties may exhibit logical features, for example, by being transitive, symmetric, inverse and functional. Properties may also have domains and ranges. Datatype properties Datatype properties are relations between instances of classes and RDF literals or XML schema datatypes. For example, modelName (String datatype) is the property of Manufacturer class. They are formulated using owl:DatatypeProperty type. Object properties Object properties are relations between instances of two classes. For example, ownedBy may be an object type property of the Vehicle class and may have a range which is the class Person. They are formulated using owl:ObjectProperty. Operators Languages in the OWL family support various operations on classes such as union, intersection and complement. They also