SOUND-Board, the principal part of an organ, and that which makes the whole machine play. It is a reservoir into which the wind, drawn in by the bellows, is conducted by a port-vent, and thence distributed into the pipes placed over the holes of its upper part. The wind enters them by valves, which open by pressing on the keys, after the registers are drawn, by which the air is prevented from going into any of the other pipes, besides those in which it is required.