Your keyboard pictures have me puzzled.
On Willems A4 is to the right of middle C whereas in Tom's A4 is to the left of middle C.
So why the difference?
Which is correct?
I have googled for other explanations but have met with another brick in the wall, as I do not
know which source on the internet to be correct!
In the Stringed Instrument Tunings link below, Guitar Standard tuning makes no reference to A4..... have a look
and see if you can enlighten me?
Dermot
Link: Stringed Instrument Tunings
Click To Enlarge Image Which Has Been Taken From The Link Above
