I understand that having fret markers spaced every two to three frets is an obvious choice: it also allows for relatively even spacing and only 5 markers per octave, which is the highest number of unordered/structured objects most humans can instantly recognize (making it intuitively very functional). My question is just about the choice in frets where these markers are placed. The octave seems like a given, but some instruments use 3,5,7,10 (like the ukulele), and others even place a marker on the first fret. 3,5,7 seem to be extremely common, though. Is there any particular reason why we put markers on frets 3,5,7,9 instead of something like 2,4,7,9 or 2,5,8,10?