It is possible to define a set of entities and the relationships among them in a number of different ways. The main difference is how we deal with attributes.

Consider the diagram to the right. It shows the entity set "Employee" with attributes "employee-name" and "phone-number".

We could argue that the phone be treated as an entity itself, with attributes "phone-number" and "location". Then we have two entity sets, and the relationship "EmpPhn" defining the association between employees and their phones.

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Source: http://www.cs.sfu.ca/CC/354/zaiane/material/notes/Chapter2/node4.html