Since the ancient Greeks, the development of mathematics has been intertwined with the study of music, which has often been the impetus for mathematical discoveries. When Jean d'Alembert published a paper on vibrating musical strings in 1747, it was simply known as the Wave Equation, our seventh work in the máthēma collection. At the time of his discovery, he could hardly have guessed that he was laying the groundwork for twentieth-century theoretical physics, and especially the Theory of Relativity. Besides his many contributions to mathematics, d'Alembert was one of history's great polymaths; a Latin scholar, philosopher, and music theorist who co-authored the world's first encyclopedia together with Denis Diderot, writing over a thousand entries himself. Friedrich the Great, king of Prussia, even proposed naming a Venusian moon after him, an honor d'Alembert refused, a smart choice as it turned out the moon did not actually exist.