A convenient graphical representation of input-output behavior of a system or subsystem, where the signal into the block represents the input and the signal out of the block represents the output. The flow of information (the signal) is unidirectional from the input to the output. A complex system, comprising several subsystems, is represented by the interconnection of blocks that represent the subsystems. The primary use of the block diagram is to portray the interrelationship of distinct parts of the system.