For example, an OrderDetail is a line item of each Order . A navigability arrow on an association shows which direction the association can be traversed or queried. An OrderDetail can be queried about its Item , but not the other way around. Associations with no navigability arrows are bi-directional. The multiplicity of an association end is the number of possible instances of the class associated with a single instance of the other end.
Multiplicities are single numbers or ranges of numbers.
In our example, there can be only one Customer for each Order , but a Customer can have any number of Orders . This table gives the most common multiplicities. The notation n . . Every class diagram has classes, associations, and multiplicities. Navigability and roles are optional items placed in a diagram to provide clarity.