A front fallboard, a rear fallboard and a hinge connected between them form in combination a fallboard assembly connected between side arms of a piano cabinet by means of a pair of rotary connectors, and each of the rotary connectors includes a front metal plate connected to the side surface of the front fallboard, a rear metal plate connected to the bottom surface of the side art defining a recess and a rotary damper; since the recess is wider than the rear metal plate already combined with the rotary damper, the rear metal plate is movable on a virtual plane defined by a fore-and-aft direction and an up-and-down direction; while a worker is seeking the optimum positions of the rotary connectors, self-tapping screws keep the rear metal plates in the recesses so that the worker drives woodscrews into the side arms through main holes or auxiliary holes.