Another slightly different google search.
what middle C standard do most musicians use C3 or C4 or C5?

Paydirt! (right near the top of that search)
We will follow the International Standards Organization (ISO) system for register designations. In that system, middle C (the first ledger line above the bass staff or the first ledger line below the treble staff) is C4. An octave higher than middle C is C5, and an octave lower than middle C is C3.

So now I have a suggestion for PG-Music. Make C4 the default option in the popup and make sure you mention in the popup that C4 is the International Standards Organization (ISO) option (protect yourself from the darts and contribute to the benefit of all). I think international means the whole world standard. So now I think I have my best guess as to why all the tuners are using C4. The phone tuners are more recent and more likely to use the ISO standard. Europe has been around a longer time. More likely have a greater number of people do not want to learn new things. However I have not taken the time to read any hits in this search. why do some musicians use C5 or C3 rather than C4?. Am I jumping to conclusions when I think it is because they just do not want to be inconvenienced and learn something new for the good of all? I don't know. I do know that most people do not like being inconvenienced.

Last edited by bowlesj; 12/19/20 07:43 AM.
John Bowles
My playing in my 20s:
https://www.reverbnation.com/johnbowles