A “constant strength” spectrum is obtained when the system’s strength is constant regardless of the natural period. Different definitions of the strength (absolute or relative to a spectrum or to elastic response), lead to several variations on the “constant strength” spectrum, such as the “constant R” spectra, where R is the force reduction, or the ratio of the linear force demand to the yield strength, and the "Constant Normalized Strength" spectra. The linear demand can be either the linear spectrum of the same earthquake or some idealized linear spectrum (for example, a design spectrum).