@b4c50e1b 

Starting from ignorance on my part except for making junk guitars for a couple of years, it amuses me how simple things quickly are made complicated. I put only 4 marks on my fretboards ... halfway (Octave), 2/3 (perfect 5th), 3/4 (perfect 4th). The 4th mark equates to minor 3rd ... where the 3rd fret on a fretted guitar would be.

Thus the dreaded tritone sits happily between the perfect 4th and perfect 5th.