The 100 tooth, MOD 0.8 gear will mate with any MOD 0.8 gear, but when building on pattern, the most common partner for this gear is a 20 tooth pinion gear or servo gear.  Using the pinion gear you can create a 5:1 underdrive or overdrive setup depending on your application.  When used with the 20 tooth servo gear, the combo creates a 5:1 underdrive, slowing the final speed while increasing the torque by 5x.