Edwin Hubble sub-categorized spiral galaxies based on the tightness of their spiral arms and the presence or absence of a central bar. He came up with the following classification:

Sa: These are tightly wound spiral galaxies with a large, prominent bulge and smooth, continuous spiral arms. They often have a well-defined bar structure.

Sb: These are loosely wound spiral galaxies with a smaller bulge and more clearly defined spiral arms. They may or may not have a bar structure.

Sc: These are very loosely wound spiral galaxies with a small bulge and fragmented, poorly defined spiral arms. They often have a pronounced bar structure.

SBa, SBb, SBc: These are the same as Sa, Sb, and Sc, respectively, but they have a prominent central bar structure.

Hubble also recognized a special category called barred spiral galaxies (SB), which are spiral galaxies with a central bar-shaped structure.

This classification system is known as the Hubble sequence and is still used today as a basic framework for understanding the morphology of galaxies.

It's important to note that Hubble's classification was based on visual observations and was initially designed to be relatively simple. As our understanding of galaxies has advanced, more sophisticated classifications have been developed that take into account additional factors like the galaxy's mass, luminosity, and stellar population.