  Spiral structure is the most distinctive feature of disk galaxies and yet
debate persists about which theory of spiral structure is the correct one. Many
versions of the density wave theory demand that the pitch angle is uniquely
determined by the distribution of mass in the bulge and disk of the galaxy. We
present evidence that the tangent of the pitch angle of logarithmic spiral arms
in disk galaxies correlates strongly with the density of neutral atomic
hydrogen in the disk and with the central stellar bulge mass of the galaxy.
These three quantities, when plotted against each other, form a planar
relationship which, we argue should be fundamental to our understanding of
spiral structure in disk galaxies. We further argue that any successful theory
of spiral structure must be able to explain this relationship.
