Abstract: A gear reduction system is provided including two-stage gear reducers that may be converted to three-stage gear reducers by addition of a modular input stage. The modular input stage converts the input stage of the two-stage reducers to an intermediate stage of the three-stage reducers. Gearing, preferably including both drive pinions and driven gears, of the input stages of the two-stage reducers are used in the intermediate stages of the three-stage reducers, thereby reducing the overall number of parts needed for a family or series of reducers. Gearing is also common between input (and intermediate) stages of smaller gear reducers and modular input stages used with larger gear reducers, further reducing the number of different parts. The two-stage reducers and modular input stages of the three-stage reducers are preferably driven by modular transmission adapters that support the drive pinions of the input stages.