The Notes Toolbar has three controls (spin boxes): Channel, Base Octave and Transpose.
You can think of the Base Octave and Transpose controls as they do a similar thing: adding a quantity to the piano key number, to calculate the MIDI note number. The first piano key has the number 0, the second is 1 and so on…
The first MIDI note is also 0, which means C-1 in Scientific Pitch Notation.
The Transpose control adds or subtracts a number to the key number, effectively implementing the chromatic transpose.
The Base Octave adds (only a positive number of) octaves to the key number: octaveBase*12 + piano key number = MIDI note.
Currently, the Transpose control is limited to a range from -11 to +11, while the Octave Base range is 0 to 9.
If you think of it, it would be possible to remove the Octave Base control, and provide only a Transpose control with the range from 0 to 120, but I think this would be more difficult to use than the current combination of two controls.
Another funny experiment: set the Octave Base = 0. Now, change the Transpose value to values between -1 and -11. See the keys below C-1 disappeared from the virtual piano! This is the complementary effect of setting the Octave Base = 3 or more, and a high number of keys.
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