Open G Taro Patch tuning (L-H: DBDGBD) obviously evolved from standard Spanish guitar tuning (L-H: EADGBE). Open A tuning evolved when guitars became purpose built for higher steel string tensions (L-H: A,C#,E,A,C#,E). These tunings were well suited to early Hawaiian vocalists, including falsetto voices. Jerry Byrd's C6 tuning was introduced during the late 1930's Western Swing era, and the highest string was typically E (L-H: C,E,G,A,C,E). C6 tuning facilitated playing within a "slot", and requiring less bar movement made playing more fluid, once the intonation of slants was mastered.
In 1934, when Rickenbacher began producing the frypan in earnest, their sales brochures touted: "Imagine a Hawaiian guitar you could hear from a quarter mile away!" Their 7-string model, available at a modest additional cost, was purposely intended to provide a 7th, because 7th cords are so common in popular music. In A6 tuning, that could be (L-H: Bb,C#,E,F#,A,C#,E); or in C6 tuning (L-H: Bb,C,E,G,A,C,E). Of course, there are many other possibilities too.
Many of us are visual learners. Maps, graphs, charts, etc. help us learn by filtering out "noisy data" and clarifying the logic of an idea or concept. I think fretboard maps can be especially useful if you're learning a new tuning, or if you want to explore alternate ways to play a melody, phrase or lick in a familiar tuning.
