C6/A7 is a six string steel guitar tuning invented by Jerry Byrd in the late 1930s. It's a variation of his popular C6 tuning arrived at by raising the low string a half step from C to C# for (low to high) C#, E, G, A, C, E. This gives the player the ability to play major, minor, 6, m7 and dominant 7 chords with a straight bar (no slants).