In Hubble's original scheme , 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  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 , who introduced ``late'' and ``early'' subtypes for the Sb and Sc galaxies, see the following table. Shapley  extended Hubble's classification to include explicitly much more ragged spirals, type Sd; while de Vaucouleurs  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).
|Sb-||Extended nuclear region representing a
considerable fraction of the total luminosity.|
Mean surface brightness is comparatively high.
Symmetrical and rather closed system of spiral arms with no pronounced contrast between the arms and the main body.
In most cases there is no appreciable resolution.
|Sb+||Comparatively small nuclear region.|
Mean surface brightness lower than Sb-.
Arm system is open and symmetrical, with good contrast against the main body.
|Sc-||small, sometimes semi-stellar nucleus.|
Mean surface brightness about the same as for Sb+.
More or less symmetrical, open and rather pronounced spiral arms.
Resolution well advanced.
|NGC 3992 |
|Sc+||No prominent nuclear region.|
Mean surface brightness less than for Sc-.
Confused and loosely defined spiral arm system (short arms).