Swell box

In an organ, "Swell" (German: "Schwellwerk;" French: "Récit") refers to the division whose pipes are enclosed in a swell box. This box has a large opening covered with moveable shades or shutters which resemble heavy venetian blinds. When open, these shutters allow the pipes' sounds to travel freely from the box to the room. When closed, most of the sound is contained in the box. Thus the moveable shades provide a means of adjusting the loudness of the sound, and perhaps more importantly, of gradual crescendo ("swelling") and decrescendo.