A square wave jumps between two values and stays flat. Fourier showed it is built from sine waves alone: add the odd harmonics 3, 5, 7 and so on, each one that many times faster and that many times smaller. Every circle here is one harmonic and the last tip's height is the wave. With 1, 3, 9, 30 and 100 circles the tops flatten and the edges sharpen, but a 9 percent overshoot never goes away: the Gibbs phenomenon. Pitch follows the height.