An important early tuning system is Pythagorean tuning, where we force all frequency ratios to involve only powers of 2 and 3. In music, 3/2 is the ‘fifth’: the most consonant of intervals except for the octave.
This is a companion discussion topic for the original entry at https://johncarlosbaez.wordpress.com/2023/10/07/pythagorean-tuning/