When you hear the very first seconds of The Beatles' "Let It Be" or Debussy's "Clair de Lune," you'll know what songs they are by their famous first chords. But what makes those chords, or groups of notes played together, sound so wonderful? Why do some chords sound not as pleasant as others? The answer lies in math. Centuries ago, the Greeks found out that the ratio of string lengths to each other determine "good" and "bad" sound: "If the lengths of the strings were in ratios of small whole numbers, such as 2:1 (an octave), 3:2 (fifths), or 5:4 (a third)" then that is when chords made by strings will sound "pleasant." What's interesting to see is that if you look at a guitar, you'll notice that the string's overall length, thickness, and even tension are different from eachother. These differences, along with the proper string setup of course, are all related to math, and are what help to make instruments such as the guitar make such beautiful music! What a wonder math is!