A stereo cartridge has 2 coils, mounted at 90 degrees to each other, and the assembly tilted at 45 degrees to the vertical. A dual-coil mono cartridge has 2 coils, mounted in the same plane, and the assembly mounted vertically. A single-coil mono cartridge has one coil (of course), mounted vertically. The point of a dual-coil mono cartridge is for wiring compatability and galvanic isolation between the two input channels of the preamp. (If you're being pedantic, substitute "set of coils" for each occurrence of "coil" above.) Someone mentioned vertical stylus movement on a worn mono disc. Vertical movement occurs on all mono discs. It is caused by the "pinch effect", one of the causes of IGD.