English: Animation of two gear wheel with involute toothing; Two involute gears, the left driving the right: Blue arrows show the contact forces between them. The force line (or line of action) runs along a tangent common to both base circles. (In this situation, there is no force, and no contact needed, along the opposite common tangent not shown). The involutes here are traced out in converse fashion: points (of contact) move along the stationary force-vector "string" as if it was being unwound from the left rotating base circle, and wound onto the right rotating base circle.