I am curious about something. I read about instruments being tuned to a note followed by a number. For example, G-2 for a bass guitar or double bass. How is the 2 derived exactly? Is it based on the piano, with the leftmost key being, for example, C-0 or C-1? I understand that it is an octave number. But, what I am not sure of is how it is derived (as I am guessing that it is not "random").