In the twentieth-century, musical theories using mathematical proceedings are different from those of the Renaissance or the Baroque period. Musicians or philosophers such as Mersenne, Descartes, Rameau, D"Alembert, Euler were essentially concerned with the problem of finding out the numerical relationship hidden in music or in sound. Their basic assumption was the direct link between the use of ratio 3/2 or a limited number of simple intervals in the ground bass and a possible rational explanation of music.