In my experience, a 7-course lute works fine for all the 6-course repertoire. It is easy enough to ignore the 7th course.
In the 7-course repertoire, sometimes there is a choice to make between D or F for the 7th course. Most often, I can finger the F (d) on the 7th course anyway, to save re-tuning for those pieces. The 8-course lute.Isn't this just a modern idea to satisfy lute-players of all 7-course tablature? If the evidence shows very little was written for the 8-course, maybe it is. A 10-course lute is my next choice instead of 8. Even then, I have come across pieces where only the open 10th appears in the bass. So maybe this is also common for 9th & 8-course tablature, where that course makes only a rare appearance. Sometimes a piece needs to be played through to determine which bass-course will work to the one indicated in the tablature. I have been playing some 10c Vallet music lately. Twice where the 8th course (/a) is used, it will not work. But I can easily substitute the 10th (///a) where that occurs. Either it is a typo, or just the 8th course needs a different tuning. 7th, 9th & 10th, no problem. Ron (UK) -- To get on or off this list see list information at http://www.cs.dartmouth.edu/~wbc/lute-admin/index.html
