The harmonic series runs up — 2×, 3×, 4× the fundamental. The undertone series is its mirror image, running down: the note divided by 2, 3, 4… Divide a pitch by a whole number and you get a lower note that's still consonant with it, which is how the Trautonium and Moog's Subharmonicon build fat, just-intoned chords from a single played note. Dial in a divisor per oscillator and watch the spectrum grow downward from the note you play.
play with your keyboard (a s d f …) or click the keys · the chord grows below the note