In an iron back, the linings only are to be glued, the remainder being fastened with screws from behind, or with small bolts, the heads of the screws being insulated with a felt button before "marking off;" the bridges must be planed to proper height, so that the down bearing of the strings may not be too great; at the treble the plank bridge and lower bridge are level, and the straight edge laid across them should be 1/2in.* above the bent side or pinning edge, the remainder of the lower bridge being so regulated that straight edges laid on the bridge and touching the pinned edge should be of the same angle everywhere as the one at the treble; the plank bridge can then be planed to the same thickness throughout, which should be about 5/8in.;* the top and bottom scales are now placed in their respective positions.
It may be necessary to explain that these are 1/4in. slips of wood, with holes bored at regular distances, those of the top scale corresponding with the hammers, those in the other rather closer together, or, as it is termed, "contracted." For convenience of reference they are also lettered to the gamut in use, a pin is placed in each hole consecutively, and with the straight edge leaning against them; a line is ruled across the bent side, lower and top bridges, this line representing the right-hand string of each note.