ructure on an organ with several manuals

The first organs had only one manual with few keys and accordingly only few pipes. In the late Middle Ages the church organ developed into its present form in Europe - particularly in Germany, Belgium and The Netherlands -, the "Great" organ (or "Hauptwerk") being fixed to the wall and another Rückpositiv (Positive / Choir) organ sitting between the player's back and the balustrade of the gallery. Thus the organ consisted of two divisions each of which was worked by its own keyboard (manual). Then also a keyboard worked by the feet (pedal) was added. As early as in the 16th century even more divisions were added to the organ, e.g. a "Brustwerk" (chest division) or an "Oberwerk" (upper division at the top of the "Great"), each having its own manual. So large organs had three to four manuals, the largest organs today have up to seven manuals.

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Note added at 8 mins (2012-03-01 09:58:57 GMT)
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based on research (no expert knowledge of organs) but this ref convinces me ...

http://www.die-orgelseite.de/funktionsweise_e.htm



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Note added at 10 mins (2012-03-01 10:01:08 GMT)
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and should read "structure" at the top of course