We noticed the other day how QAM (quadrature amplitude modulation) bit rates, e.g., in optical fibre, increase by 4x at each step: QAM16, QAM64, QAM256. The Wikipedia page makes why this is completely clear. In the digital version of QAM there’s an NxN matrix, a “constellation” of points when viewed as an amplitude and phase angle. Scroll down to “Quantized QAM” on the page to see the animated GIF on the right.