in music, a set of pipes in an organ with a separate key-board, and forming a separate department, which are capable of being increased or diminished in intensity of sound by the action of a pedal on a series of shades or shutters overlapping each other like Venetian window-blinds, within which the pipes in question are enclosed. On a well-constructed swell a practiced performer can imitate not only a gradual crescendo and diminuendo, but also a forzando, a very small opening sufficing to make an immediate burst upon the ear; while, when the shutters are closed, an imitation of an echo is produced.