harmonics

Andrew Huang explains the harmonic series, which he calls the foundation of almost all music. He builds the idea up step by step, with demonstrations of overtones and how harmonies derive from them.

A minutephysics video explaining why a piano cannot be perfectly tuned, comparing equal temperament with just intonation and the math of harmonics.

A short demonstration comparing just intonation with equal temperament, played so you can hear how the tuning compromise changes intervals and chords. It includes a Bach prelude played in both tunings.