The reason underneath
Every note is a chord already.
In 1636 CE Marin Mersenne listened closely to one plucked string and heard quiet extra tones ringing above it. In 1701 CE Joseph Sauveur measured them: a string vibrates whole, in halves, in thirds and onward, all at the same time.
This museum calls each pure component a partial, numbered from 1 at the fundamental. Sauveur named the whole-number multiples harmonics and coined the word acoustics. An older name, overtone, skips the fundamental, so overtone 1 is partial 2.
This explained the ancient pattern. Two notes an octave or a fifth apart share many of these partials, so they fuse. A rough pair sets neighbouring partials beating against each other, a mechanism Hermann von Helmholtz worked out in 1863 CE.
Even the major triad was waiting inside the string: partials 4, 5 and 6 of any deep note spell it out. The small-number ratios the Greeks measured on the monochord match the spacings between one string's partials.
Play the deep string, climb partials 1 through 6, then hear partials 4, 5 and 6 as a triad. Listen for the staircase to rise while the last three tones fuse into one chord. A string vibrates in whole-number sections at once, creating these partials. Their shared pattern connects the ancient ratios to the physical structure inside every periodic musical tone.
From the lobby, visit any exhibit room to hear its discovery more closely.