Standard open-end wrenches only contact two corners of a nut. When you try to loosen a rusted brake line or fuel fitting, the soft brass or steel nut will easily round off. A flare nut wrench, also called a line wrench, solves this problem. It looks like a box wrench with a small slot cut out of the ring. This slot allows you to slip the wrench over the metal tubing and slide it down onto the hex fitting.
Once in place, it contacts five or six corners of the nut, distributing the force evenly. This lets you apply high torque without slipping or crushing the soft fitting. Clean the fitting with a wire brush and apply penetrating oil before trying to loosen it. If the nut is badly rusted, tapping the wrench gently with a brass hammer can help break the threads loose without damage.