Sandy asked what keys are represented by the 0-12 and 0+12 settings. As Dr. Einstein said, "It's all relative." When the transpose setting is at 0 then playing middle C gives you middle C. So whatever key you play in will be "true" and accurately tuned. If you go up one, for example (+1), then playing middle C will sound a C#. Lowering the transpose setting one (-1) will sound a B if you play middle C. This way you can transpose to hear the music in any key no matter what key you're playing in. I use this all the time if singing in the original key is difficult for a soloist at our church where I play.
