Spur and Helical: offset or planetary arrangements, which can be analyzed either as speed reducers or speed increasers. Either Rack or Shaper cutters, with crowning and profile modification up to 4th order.
Double-helical/Herringbone gears are helical gears with opposed helix angles on each half, thereby eliminating axial reactions. Teeth may be aligned or staggered.
Beveloid gears are spur or helical gears with or without intersecting axes, depending on the sign of their pitch angle. One or both of the members being conical, an axial displacement modifies the backlash. Can be Internal/External.