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On Proportional Rhythm in Chant:

All the longs must be equally long, all the shorts of equal brevity.... In accordance with the length durations let there be formed short beats, so that they be neither more nor less, but one always twice as long as the other. - Commemoratio Brevis, early 10th cent.

In the neumes it is necessary that you pay close attention where the proportional shorter duration is to be measured and where, on the contrary, the longer duration, lest you execute as quick and short what the authority of the masters has determined should be longer and more extended. Nor should we heed those who say there is no reason whatsoever for our making now the quicker duration, now the more prolonged one, in a chant with a naturally disposed rhythm. - Berno of Reichenau, early 11th cent.

In olden times great care was observed not only by the composers of the chant but also by the singers themselves to compose and sing proportionally. - Aribo, late 11th cent.