Has anyone heard of this? I think it goes like this:

If you have music in (e.g.) the key of Ab (4 flats) and would like to play in an easier key, you can subtract the number of flats from 7 and play the piece AS WRITTEN as if it were in the key of 3 sharps, or A major, in this case. Similarly, Eb (3 flats) subtracted from 7 would translate easily to E major (4 sharps). You would need to remember what's supposed to be sharp in your new key, of course.

This came up when I was asked to play fiddle at a church service to accompany a bass and piano, using the church hymnal. Being a perennial beginner, I had trouble with some of the keys with lots of flats and sharps. The pianist (the music director) told me about the "Rule of 7" and it went very well. It was especially useful to me as a simple fiddler, being able to go to good fiddle keys on some of those unfamiliar hymns, although the congregation probably wondered why they had to stretch to reach some of the notes, no doubt.

The music director is no longer around to ask, and I wonder if I remember it correctly. I just tried out Eb and Ab, but can't find any music in other heavily flatted or sharped keys to verify the rule.

Hey, I just noticed that the key stays the same with the "Rule of 7" while it's just the symbol -flat or sharp- that changes.

Is this a well-kept secret? Or am I the only one who didn't know this before?