An arity of two is also known as binary.  A binary relationship, the most common on a traditional data model diagram, involves two entities.  The Binary Relationship figure, a UML class diagram, shows that both Student and Course are entities participating in a binary relationship.

An arity of three, known as ternary, is a relationship that includes three entities.  An example in fact-based modeling (object -role notation) appears in the Ternary Relationship figure.  Here Student can register for a particular Course in a given Semester.