As nickedshield notes, the green crosshairs represent the origin and the red crosshairs represent the endpoint.  The endpoint is mainly used for IK and PointAt.  When you get close to the center of the green crosshairs, you should see a circular 'target' mouse cursor.  Click and drag then to move the crosshairs.  If you want real accuracy, set the Document Window to display orthographic views.

C makes it easy to shoot yourself in the foot. C++ makes it harder, but when you do, you blow your whole leg off.

 -- Bjarne Stroustrup

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