De institutione musica II.13: De continuis medietatibus et disiunctis.
Among these, one proportionality is continuous, another disjunct. The continuous is as we set it out above; for one and the same middle number is now placed under the greater, now set before the lesser. Whenever there are two middles, it is called disjunct proportionality, as in the geometric in this manner: 1, 2, 3, 6. For as two is to the unit, so is six to three; and this is called disjunct proportionality. Whence it can be understood that continuous proportionality is found in three terms at the least, disjunct in four. Continuous proportionality can also be in four and more, if it is in this manner: 1, 2, 4, 8, 16. But here there will be not two proportions but several, always one fewer than the terms set.