  We study the long term evolution of the relationship between the root mean
square (rms) variability and flux (the ``rms-flux relation'') for the black
hole Cygnus X-1 as monitored from 1996 to 2003 with the Rossi X-ray Timing
Explorer (RXTE). We confirm earlier results by Uttley & McHardy (2001) of a
linear relationship between rms and flux in the hard state on time scales > 5 s
reflecting in its slope the fractional rms variability. We demonstrate the
perpetuation of the linear rms-flux relation in the soft and the intermediate
state. The existence of a non-zero intercept in the linear rms-flux relation
argues for two lightcurve components, for example, one variable and one
non-variable component, or a possible constant rms component. The relationship
between these two hypothesized components can be described by a fundamental
dependence of slope and intercept at time scales ~< 10 ksec with long term
averages of the flux.
