In a series of reduction gears including a parallel shaft reduction gear and an orthogonal shaft reduction gear, the parallel shaft reduction gear includes a motor, a parallel shaft reducer, and a first joint unit, and the first joint unit includes a first joint casing which is connected to a motor casing and a parallel shaft reducer casing, and a joint shaft which is connected to a motor shaft and the input shaft of the parallel shaft reducer. The orthogonal shaft reduction gear includes the motor and the parallel shaft reducer which are used in common with the parallel shaft reduction gear, a second joint unit in which a portion or all portions are used in common with the first joint unit, and an orthogonal reducer, and the motor, the second joint unit, the orthogonal reducer, and the parallel shaft reducer are connected to one another in this order.

A bidirectional gear set includes a male bidirectional gear component having an array of addendum teeth and a female bidirectional gear component having an array of dedendum sockets, wherein each dedendum socket has a full boundary edge around an open end thereof. One or both of the male and female gear components may have a circular cross sectional geometry, or one may have a non-circular cross sectional geometry. The non-circular cross sectional geometry may be a flat sheet or a sheet with surface contour. In an intermeshed state, neither the male nor the female component can slide relative to the other regardless of whether the male and female components are arranged in a parallel or perpendicular orientation. A method of making a bidirectional gear set is disclosed.