Why guitar frets get closer together as you go up the neck

Guitar frets crowd together toward the body because of one fact about hearing: we hear pitch by ratio, not by difference. Each fret has to raise the note by the same factor, about 1.0595, so it cuts off about 5.6% of whatever string is left, not a fixed distance.

That's why the 12th fret sits exactly halfway along the string, and why the gaps halve every 12 frets. Tap the neck to hear what goes wrong with evenly spaced frets, and why the old luthier's "rule of 18" is only almost right.