I believe that the 7-string guitar should have the same range as the 8-string and 10-string guitars. This can be achieved by tuning the seventh string to A. In 8-string guitars, the tuning would be A and D, and in 10-string guitars, A, B, C, and D.
The seventh string can also be tuned to G if we are going to play in C, as in the Prelude BWV 846 from the Well-Tempered Clavier.
[link text]https://youtu.be/57UdYQjBtYA(link url)
