@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.
@8a3ca29f - things get really complicated! In the Pythagorean scale here we get two different notes: the augmented 4th which is a bit lower than the tritone, and the diminished 5th which is a bit higher. The augmented 4th is 1024/729 ≈ 1.404664 the tritone is √2 ≈ 1.414214 and the diminished 5th 729/512 ≈ 1.423828 I sure don't claim I can hear the difference! But if two were played simultaneously they would sound out of tune.