Most organs have more ranks than stops. This means that some (like mixtures and other compound stops) have more than one rank (row of pipes) per stop. In other words, when you press one key, several pipes speak at the same time, though only one stop is engaged.
What happens if an organ has one stop which is distributed throughout several divisions at multiple pitch levels? That's what they call unit organ or multiplex system. This way you can have an organ of considerable size with only a handful of stops.
I played a 4 rank, 44 stop, 3 manual and pedal organ once.The 4 ranks were one from each of the 4 stop families - a principal, a flute, a viola, and a trompette.
Although such organs are much cheaper to build, the flip side of multiplication is of course that there is not so much of variety in tone color (the pipes of 16', 8', 4' and 2' principals belong to the same principal stop).
Number of ranks matter a lot more than it may seem at first.
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Organists of Vilnius University , creators of Secrets of Organ Playing.
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Don't have an organ at home?
Download paper manuals and pedals, print them out, cut the white spaces, tape the sheets together and you'll be ready to practice anywhere where is a desk and floor. Make sure you have a higher chair.