Here I show a simple mathematical description for the most common genera of pentatonic scales – the modes of the major pentatonic scale – and related the model to the harmonic series. Specifically, I describe the major pentatonic scale using a single frequency ratio of 3/2, the interval relationship found between the 2nd and 3rd partials of the harmonic series. In western terms, the 3/2 frequency ratio is called a ‘Perfect 5th’.