The Harmonic Square is a revolutionary new musical instrument conceived by theoretical physicist and inventor Nick Charlton. It has a set of 25 keys arranged in a square, each one playing a harmonic of a particular base note. This arrangement enables the player to produce a far richer selection of harmonics of a given note than an ordinary keyboard. The Harmonic Square is more like a 2-dimensional keyboard. Keys are one of two colours. The dark keys correspond to notes that already exist on a keyboard. The light keys correspond to new harmonics that cannot be produced on an ordinary instrument.