Spending time lately investigating and memorizing the myriad slants available in C6-based tunings, especially C13, and I stumbled upon this beautiful piece of musical geometry.
There are many other hidden secrets in this tuning and I aim to find and use all of them!

Jim W. says:
Hi Mike,
Great visual of the fret board for 8 string C13. Very helpful for primary neck on my National double neck. I recently became the lucky owner of an Alkire 10 string. I’m still in deliberations with myself as to what to do with the two additional strings. I feel it makes sense to add the high G to get more range (and it’s a continuation of C6th) but as to add another bass string or yet another high string I’m not sure (some people seem to add a D after the high G which seems intriguing – more range and a 9th).
Wondering what you’re thought were in the grand visualizing of the fret board.
Thanks!!!
Jim