Figure 3-10 illustrates the four basic symbols used in DFDs. A rectangle or square represents an external data source or data destination—for example, a customer. To show this, a DFD would include the word ‘‘customer’’ inside a data source or destination symbol. In Figure 3-10, the term ‘‘external entity’’ means ‘‘an entity outside the system under study,’’ not necessarily an entity that is external to the company. Thus, for example, a ‘‘customer’’might be another division of the same company under study.