In Hubble's original scheme [53], spirals were subdivided into three types or stages (Sa, Sb, and Sc) characterized either by an increasing openness of the arms or by the decreasing importance of the light contributed by the bulge compared to the disk. Even before this time Curtis [24] had paved the way for independently dividing all spirals into two families according to whether they possessed central bar structures (barred spirals) or not. These two form criteria are the basis of the so-called ``tuning fork'' diagram for galaxies; see LB, NS, Vol. VI/1, p. 668.

Extension of the basic Hubble scheme started with Holmberg [51], who introduced ``late'' and ``early'' subtypes for the Sb and Sc galaxies, see the following table. Shapley [171] extended Hubble's classification to include explicitly much more ragged spirals, type Sd; while de Vaucouleurs [27] further extented the system to those galaxies bordering on the irregular classification - having no nucleus and only a hint of spiral structure - type Sm (spirals of Magellanic Type).