De institutione musica II.11: Qui superparticulares quos multiplices efficiant.
To these it must be added that the first two superparticulars make the first multiple proportion. As if the sesquialter and the sesquitertian are joined, they create the double. Let the numbers be 2, 3, 4. Three to two is sesquialter, 4 to 3 sesquitertian, 4 to 2 double. Again, the first multiple added to the first superparticular makes the second multiple. Let the numbers be 2, 4, 6. For four to two is double, the first multiple; six to four is sesquialter, the first superparticular; six to two is triple, which is the second multiple. If you add the sesquitertian to the triple, the quadruple will be made; if the sesquiquartan to the quadruple, the quintuple; and in this manner, by joining proportions of multiples and superparticulars, multiples are begotten without limit.