The time progresses as you go down the page. Sequence diagrams are organized according to time. Sequence Diagram is an interaction diagram that details how operations are carried out - what messages are sent and when. The vertical space in an interaction diagram is not relevant for the duration of the interaction. Time in a sequence diagram is all a about ordering, not duration. The vertical axis represents time proceedings (or progressing) down the page.However, the elements on the horizontal axis may appear in any order Conventionally, the objects involved in the operation are listed from left to right according to when they take part in the message sequence.The horizontal axis shows the elements that are involved in the interaction.Sequence Diagrams show elements as they interact over time and they are organized according to object (horizontally) and time (vertically): Object Dimension

Either model generic interactions (showing all possible paths through the interaction) or specific instances of a interaction (showing just one path through the interaction).

Model the interaction between objects within a collaboration that realizes an operation.

Model the interaction between object instances within a collaboration that realizes a use case.Model high-level interaction between active objects in a system.

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