A gear joint is used to connect two joints together.

Either joint can be a revolute or prismatic joint. You specify a gear ratio to bind the motions together: coordinate1 + ratio * coordinate2 = constant

The ratio can be negative or positive. If one joint is a revolute joint and the other joint is a prismatic joint, then the ratio will have units of length or units of 1/length.

RESTRICITON: The revolute and prismatic joints must be attached to a fixed body (which must be body1 on those joints).