>>40829580
Get a cheap stringed instrument or borrow one.
Gently touch the string at the half way point and pluck it (this may take some practice to get the touch right.) this will produce a tone at double the frequency of the whole string. In western music this is called an "octave." This is the first harmonic. Do it one third of the way down it will make a tone that's called a "fifth" because in western music is the fifth note in the scale, but that's arbitrary. The interval between this "fifth" and the next doubling is called a fourth since it's the same as the fourth note in the scale, but there's no simple fraction of the string to make this.
What's more interesting than all of this is that the so called "cycle of fifths" taught in music theory is a lie. This doesn't exist in nature and if you try it with real ratios you'll never get back to the frequency you started. Modern music is chopped to make it line up. This is why pianos never sound as good as violins or human voices. In order to make all the different keys line up they split the difference and hid it. Major thirds are to big. Minor thirds are too small.
This is also why Kieth Richards sounds good on any of his open G tuned tracks. His guitars are tuned exactly to the key he's playing in and not the equal temperament bullshit a regular EADGBE guitar is tuned to. He also gets rid of the thick top string to eliminate the vibrations of the low fifth. There's nothing special about it, it's a 6 string guitar with only 5 strings tuned to GDGBD, and to itself so it's in perfect harmonics. The frequencies are in even ratios and sound better.
If you've ever heard someone try to play honky tonk woman on a standard tuning you'll see what I mean.
