The 7-string guitar is popular in Russia and Brazil.
The scale is 650 mm and the tuning is: e h g D A E H
Alternative tuning on the 7-string is: D C or A
The 8-string guitar is like a 6-string guitar with 2 extra bass strings.
The scale is 650 mm and the tuning is e h g D A E D A
The 10-string guitar, also known as the laudarra, was invented in 1960´s by Yepes in collaburation with the guitar maker Ramirez. The 10-string guitar makes it possible to play music written for baroque lute.
The scale is 650 mm and the tuning is: e h g D A E D C H A
Alternative tuning on the 4 bass string is: C A# G# F#
13-string dm guitar. Tuned like a baroque lute. The scale on this guitar is 640 mm on the 9 fingerboard strings.
The tuning is f d a F D A G F E C H A.
Alternative tuning like a regular guitar in e. This require a different set of strings.