Abstract: In the art of scaffolding, temporarily utilizing elevated planks, stages or platforms for supporting workmen and materials through use of two or more leaning ladders, a compact ladder jack has a tubular, scaffold-supporting arm for storing a suspension chain when not in use. The arm hooks over a ladder rung and the chain is looped over a higher rung. The chain has a snap hook for receiving a preselected chain link to permit mounting of the jack in a position where the arm is disposed horizontally to accommodate variances in ladder inclinations. A latch on the arm guards against accidental displacement of the hook end of the arm from the lower rung.