2026-07-072026-07-07http://salesiana.dossiersoluciones.com/handle/123456789/1695I describe the disk mass-rotation velocity relation which underpins the familiar luminosity-linewidth relation. Continuity of this relation favors nearly maximal stellar mass-to-light ratios. This contradicts the low mass-to-light ratios implied by the lack of surface brightness dependence in the same relation.4 pages including 1 figure. Invited review for "Galaxy Disks and Disk Galaxies", eds. J.G. Funes S.J. & E.M. CorsiniAstrophysicstext