Abstract: A rig mast torque stabilization system has a racking board extending rearward from a middle portion of a vertical mast. Right and left braces are movable from a transport position to an operating position extending laterally outward from the mast. Right and left mast tethers are each connected to the corresponding brace, and to the mast above the brace. Right and left board tethers are each connected to the corresponding brace, and to the racking board. Right and left vehicle tethers are each connected to the corresponding brace, and to a forward location on the vehicle supporting the mast. The mast tethers, board tethers, and vehicle tethers are configured to be in tension when the racking board is in the board operating position, the mast is in the mast operating position, and the braces are in the brace operating position.