Sequence diagrams describe interactions among classes in terms of an exchange of messages over time and are used to explore the logic of complex operations, functions or procedures. In this diagram, the sequence of interactions between the objects is represented in a step-by-step manner.

Sequence diagrams show a detailed flow for a specific use case or even just part of a particular use case. They are almost self-explanatory; they show the calls between the different objects in their sequence and can explain, at a detailed level, different calls to various objects.

A sequence diagram has two dimensions: The vertical dimension shows the sequence of messages in the chronological order that they occur; the horizontal dimension shows the object instances to which the messages are sent.

A sequence diagram is straightforward to draw. Across the top of your diagram, identify the class instances (objects) by putting each class instance inside a box (see above figure). If a class instance sends a message to another class instance, draw a line with an open arrowhead pointing to the receiving class instance and place the name of the message above the line. Optionally, for important messages, you can draw a dotted line with an arrowhead pointing back to the originating class instance; label the returned value above the dotted line.