Sequence diagrams show potential interactions between objects in the system being defined. Normally these are specified as part of a use case or use case flow and show how the use case will be implemented in the system. They include:

	Objects - oblong boxes at the top or actors, named or just shown as belonging to a class from or to which messages are sent to other objects.
	Messages - solid lines for calls and dotted lines to data returns, showing the messages that are send between objects. This includes the order of the messages which is from top of the diagram to the bottom.
	Object lifelines - dotted vertical lines showing the lifetime of the objects.
	Activation - the vertical oblong boxes on the object lifelines showing the thread of control in a synchronous system.