Cromwell, there are so many ways to mount bags, most need to have a metal frame to keep them out of the wheel or belt area. As Steve says, go to the Sportster section and you will see the gamut...from simple "throwover bag"...all the way to MzBlings' bejeweled Sportster hard bag touring setup, which Glider posted previously.
Here is a bike showing those bag support hoops...from pic of JohnCooking's ride...
The simplist are the "throwover bags" which simply have a middle yoke that drapes over the rear fender, the shock tower mount and rear turnsignal keep in centered and are secured by the seat. There are loops for lanyards on the back of each tie them to those thin wire (1/4 metal hoop supports) to prevent flapping or fouling wheel/belt area. Shown are the DragSpecialties Throwover Bags (I recently changed to Leatherneck Jim's bags that match the Mustang tank bib) I have on the bike below.
Next type is the metal plate that the bags secure to, but have keyhole or button/key catches that stay with the bags so only tiny chrome knobs show when bags are removed like the ones Smok'inToad posted earlier. The bags may be leather, with polystyrene reinforced molded back and fit very flush, and virtually all the USA made ones like Leatherworks, Leatherneck Jims, Boss Bags and others use this integral metal plates mounted bag system that key releases in a jiffy.
Finally there are the honest to gosh bagger type, with guards, lights and virtually a touring bike setup. The world is your oyster, based on what you want for your application, if you ride two-up for long distances, or just short commuting.