This UML Sequence Diagram Example is an interaction diagram that shows how operations are carried out, including what messages are sent and when they are sent. Time is used to organize sequence diagrams. As you scroll down the page, time passes. The objects involved in the operation are listed from left to right in the order in which they appear in the message sequence. The diagram below shows the steps involved in making a hotel reservation. A Reservation window is an object that starts the message sequence. In a sequence diagram, time is all about ordering rather than duration. In an interaction diagram, the vertical space is unimportant for the course of the interaction.