Gears with teeth on the internal side of the cylinder are known as "internal gears". An external gear can mesh with an external gear or an internal gear. When two external gears mesh together they rotate in the opposite directions. An internal gear can only mesh with an external gear and the gears rotate in the same direction. Due to the close positioning of shafts, internal gear assemblies are more compact than external gear assemblies. The spur gear slewing drive type is the more common one. http://www.ldb-slewdrive.com/spur-type-slewing-drive/