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 or actors at the top - either 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 for data returns, showing the messages that are sent between objects including the order of the messages which is from the top to the bottom of the diagram.
	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.