Up to this point we’ve discussed what your system will do (use cases), kind of how they will do it (activities) and what it will consist of (classes). Now we get to the point where we incorporate them together. Basically, we’ll be showing how the classes will be used in the functioning of the system. And for this we’ll use Interaction Sequence Diagrams.
Each diagram will describe a single action within your system. Do not describe your entire system in one diagram. That’s wrong.
They can be daunting at first, but are actually easy to understand once you get going.
While I normally say you should stay consistent in your design, I really mean it here. Interaction Sequence Diagrams can easily lend themselves to inconsistencies.
The one thing to remember is that Interaction Sequence Diagrams are read top to bottom. Your vertical axis (y-axis, up-down) indicates the flow of time.