Abstract Correlations between the luminosity class in the DDO system and several observational quantities are investigated for late-type spiral galaxies. The luminosity class is found to be well correlated with maximum rotational velocity log Vm (correlation coefficient –0.83), Holmberg’s (1958) photometric radius log RH(–0.80), and absolute B luminosity log LB(–0.62). The present statistical study suggests the presence of a minor but distinct extraordinary (tidal) group of galaxies; they show peculiarities in the form of their spiral arms (two strong arms for ScI’s), in the ratio of the radius of maximum rotational velocity to the Holmberg (1958) radius (Rm/RH< 0.3), and in their mass (relatively small for the same luminosity class, when evaluated by one-component disk models), which are probably related to the presence of nearby companions. As for the ordinary group, by utilizing one-component thin disk models and the density-wave theory, we infer from the above correlations that the essential properties of the high luminosity class are caused by the general increase of the scale radius R0 and/or of the scale surface density μ0. The distance-independent quantity Vm is found to be the best directly observable quantitative parameter characterizing the luminosity class.