  We consider the four large optical rotation curve samples of Mathewson, Ford
& Buchhorn (900 ORCs), Mathewson & Ford (1200 ORCs), Dale, Giovanelli, Haynes
et al (494 ORCs) and Courteau (300 ORCs).
  We show how it is possible to partition any given ORC into two distinct
dynamical regions - an interior region and an exterior region. Each of the
samples considered is partitioned in this way, so that we are able to consider
the dynamics on the exterior parts of the ORCs for each of the samples. This
technique was originaly introduced by Roscoe astro-ph/0107305.
  We then find that the dynamics in the exterior part of spiral discs fall into
one of four sharply defined classes. Taking account of the various correlations
which exist between luminosity properties and dynamical properties of discs,
our final phenomonological conclusion is that spiral discs in the samples
considered appear constrained to occupy one of four distinct planes, F(M,S,a) =
k0, k1, k2, k3, where M = absolute magnitude, S = surface brightness, "a" is a
parameter computed directly from each ORC and k0, k1, k2, k3 are well defined
numerical constants.
  These results are statistically supported at the level of virtual certainty.
  We suggest three possible mechanisms for this phenomonology. The overall
analysis implies the existence of a second generation of Tully-Fisher methods
in which the standard TF relation is augmented with a second relation defining
where on an ORC the linewidth should be measured.
