A classification of the luminosity of spiral and irregular galaxies, based on the fact that their luminosity is related to the appearance of their spiral arms, the brightest systems having the best-developed structure. Its name derives from the David Dunlap Observatory, where it was developed by the Dutch-born Canadian astronomer Sidney van den Bergh (1929– ). The DDO scheme retains the Sa, Sb, Sc, Irr notation of the Hubble classification but adds luminosity classes similar to those applied to stars, ranging from I (supergiants), via II (bright giants), III (giants), and IV (subgiants), to V (dwarfs). Class I corresponds roughly to an absolute blue magnitude of −20.5 (i.e. 2 × 10¹⁰ solar luminosities) and class V to blue magnitude −14 (10⁸ solar luminosities). No Sa or Sb spirals are fainter than class III.