Abstract: A backup tool supports a nut from rotation. The tool has a socket opening having opposing faces that lie along adjacent sides of an equilateral triangle. A threaded opening in the tool has an axis that intersects a third side of this equilateral triangle, and an adjustable screw threads into this opening. To adjustably fit the tool on a nut, the socket opening disposes around the nut, and the screw threads in the opening until the screw's distal end engages one of the six sides of a hex nut. The opposing faces along the sides of the equilateral triangle then engage two alternating sides of the hex nut. To support the nut from rotation, a reaction surface of the tool engages against a supporting surface of another nut, an edge of a flange, or the like.