The top string is a fixed open reference at A4 = 440 Hz. The red string begins open (far right). Select a preset interval to see Pythagoras’ simple ratios at work.
On a monochord, pitch rises as the vibrating length shortens. Pythagoras observed that simple whole-number frequency ratios (like 2:1, 3:2, 4:3, 5:4, 6:5) produce the most consonant intervals. Because frequency is inversely proportional to length, realizing a frequency ratio \(a\!:\!b\) on a string is done by stopping the string at \(b/a\) of its full length.