The overtone series is the group of notes that exist relative to every note. It is easiest to find on the guitar in the form of harmonics as Pythagoras discovered a few thousand years ago (not on guitar... just in general!).
If you're a serious music theorist, you can find the most basic history of harmony in music in the overtone series. The first few notes represent early music when it was mostly made up octaves and fifths. Following are thirds which we begin to hear in Medieval music and really come out in the Renaissance. Next the Baroque introduces the 7th which brings us to the Classical era. The Romantic introduces the 9th, and beyond that, the series becomes more complicated and dissonant, like 20th century music and beyond!