  We use a homogeneous sample of about 1100 rotation curves (RCs) and relative
surface photometry to investigate (out to 2 optical radii and over 6 mag) the
main mass structure properties of spirals. We confirm the strong dependence on
luminosity for both the profile and the amplitude of RCs. Spiral RCs show the
striking feature that a single parameter, the luminosity, dictates the rotation
velocity at any radius for all objects, so revealing the existence of a
universal RC. At high luminosity, there is a slight discrepancy between the
profiles of RCs and those predicted from the luminous matter (LM)
distributions: this implies a small, yet detectable, amount of dark matter
(DM). At low luminosities the failure of the LM prediction is most severe, and
DM is the only relevant mass component. Some of the derived scaling properties
between dark and luminous galactic structure parameters are: (a) the DM/LM mass
ratio at the optical radius scales inversely with luminosity; (b) the halo core
radius is comparable to the optical radius, but shrinks for lower luminosities;
(c) the central halo density scales as L^{-0.7}; (d) the halo virial mass
scales as L^{0.5}. Such scaling properties can be represented as a curve in the
luminosity-(DM/LM ratio)-(DM core radius)-(central DM density) space, which
acts as the spirals' counterpart to the FP of ellipticals.
