In the technique of mural painting, the expression indicates a particular method for marking, on the surface which is to be painted, horizontal and vertical lines of reference for the composition or the grid for the transportation of the drawing. The procedure foresees that, on the surface, a string is held tightly (between pre-defined points) usually covered with charcoal powder or brown earth: taking advantage of the tension of the string, it is lifted into the centre and then let go to fall, so that on the wall, a straight line is imprinted.