A very small organ may only have one manual (keyboard). Most organs have at least two. In English and American Organs the lower manual is the main one and is called the "Great". The upper manual is called the "Swell" because it operates pipes which are inside a “swell box” which has that can be opened or closed. This makes the music get louder or quieter (crescendo or diminuendo). The organist operates the swell box with a pedal which (rocks to and fro). It is in the centre just above the pedal board. On old English organs the swell box is operated by a lever at the side. This is quite difficult to use. Most of these have now been replaced by central swell boxes.