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An understanding of state transitions is essential in real-time software design. However, satisfactory research has not been performed on methodologies to list the necessary states. A method is needed to systematically identify necessary states in order to meet the given requirements. The two points that must be clarified when identifying necessary states are which states are necessary states and what causes lead to considering a state necessary. We begin this paper by clarifying the necessary states in software design that must be introduced into the software when elements other than state parameters external to the software arc required to determine the correct response when multiple responses are obtained from the same input. Next, by focusing on the relationship between the input and the output, we illustrate that the software requirements have a hierarchical structure and selections from alternative realization methods based on the structure cause consideration on states. This requirement structure designates the logical structure of the requirements specified in the design process. Design and specification (implementation) of the requirements make is possible to list necessary states systematically. [ABSTRACT FROM AUTHOR]