The warping or twisting of fretwork is ofttimes counteracted by the use of three-ply wood - that is, three pieces of very thin board or veneer glued together, the middle one being transverse to the others. Warping is often caused by excessive polish being applied to one side only, without a coat of varnish on the back to counteract. Nothing can afterwards be done, except to take the brackets apart and slightly damp them with clean water: screw them down between two stout boards till straight, then apply polish or varnish on both sides. There will still be the tendency to twist back again if the brackets are put in a hot place.