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Spiral structure is the most distinctive feature of disk galaxies and yetdebate persists about which theory of spiral structure is the correct one. Manyversions of the density wave theory demand that the pitch angle is uniquelydetermined by the distribution of mass in the bulge and disk of the galaxy. Wepresent evidence that the tangent of the pitch angle of logarithmic spiral armsin disk galaxies correlates strongly with the density of neutral atomichydrogen in the disk and with the central stellar bulge mass of the galaxy.These three quantities, when plotted against each other, form a planarrelationship which, we argue should be fundamental to our understanding ofspiral structure in disk galaxies. We further argue that any successful theoryof spiral structure must be able to explain this relationship.

A Fundamental Plane of Spiral Structure in Disk Galaxies