A sequence diagram illustrates object interactions arranged in a time sequence. These diagrams are typically associated with use cases. Sequence diagrams show you step-by-step what has to happen to accomplish something in the use case. This type of diagram emphasizes the sequence of events, whereas collaboration diagrams (an alternative view of the same information) emphasize the relationship. This type of diagram is best used early in the design or analysis phase because it is simple and easy to comprehend.