A 9x19mm Parabellum is a nine millimeter bullet. It is usually used in handguns or sub-machine guns. The nine millimeter is often referred to as a "nine", "9mm", or "9x19". It is one of the most widely used bullets in history originating with the 9 by 19 Luger, invented in Germany. Many guns fire the 9mm, most notably the Glock 18.