  We explore the Tully-Fisher relation over five decades in stellar mass in
galaxies with circular velocities ranging over 30 < Vc < 300 km/s. We find a
clear break in the optical Tully-Fisher relation: field galaxies with Vc < 90
km/s fall below the relation defined by brighter galaxies. These faint galaxies
are however very gas rich; adding in the gas mass and plotting baryonic disk
mass Md = M* + Mg in place of luminosity restores a single linear relation. The
Tully-Fisher relation thus appears fundamentally to be a relation between
rotation velocity and total baryonic mass of the form Md = A Vc^4.
