This example shows a sequence diagram for the buy tickets use case. This use case is initiated by the customer at the ticket vending machine communicating with the box ofﬁce. The steps for the make charges use case are included within the sequence, which involves communication with both the ticket vending machine and the credit card service. This sequence diagram is at an early stage of development and does not show the full details of the user interface. For example, the exact form of the seat list and the mechanism of specifying seats must still be determined, but the essential communication of the interaction has been speciﬁed by the use case.