Basically, belt slots fore and aft. The originals, by Roy Baker ("The Pancake Maker") of Arkansas, were two layers, the lower portion of which were almost circular. They had three slots, one above the other on the topstrap side of the gun, for either strongside or crossdraw. Roys' were a soft leather, not fancy, but serviceable for their time. The design has advanced to hard-molded rigs like Alessis' Belt Slide or Kramers' Belt Scabbard. The pancake is my favorite strongside OWB for revolver or auto - very stable on the right belt, and tucked in tight and hideable with the proper cant and slot placement. These last two items define the better rigs. The two mentioned are my best, with a nod going to Alessi (about two degrees more cant - large difference in concealment and comfort). Dillons' Master is another that benefits from some good input in this area.