When what we now call Gregorian chant was first written down around the 10th century, scribes used symbols called neumes to notate the ornate melodies. The earliest manuscripts do not give precise indications of pitch, since the notation served only as a memory aid for notes that were already committed to memory. The great variety of symbols must therefore represent a diversity of rhythm. This is the foundation of the field of study known as Gregorian Semiology.
We come to our conclusions about the rhythm of 10th century chant not by mere speculation (as skeptics of the most recent research would have you believe), but by comparing numerous manuscripts and methodically decoding the meaning of the various neumes. In some cases, however, a particular chant might only appear in only one manuscript. How, then, are we to be certain of its interpretation? Take a look at this excerpt from the mode VII gradual Audi filia sung by members of Euouae according to the notation found in the Cantatorium of St. Gall:
Since this chant does not appear in the Laon manuscript, you might think it was impossible to confirm our rhythmic reading. However, substantial portions of this melody also appear in another mode VII gradual: Salvum fac populum tuum, sung here by our friends in Lisbon, Portugal according to the three-line notation in the Graduale Novum.
Here’s another phrase that is common to both chants:
While the stylistic interpretations of the two ensembles are quite different, the rhythmic readings are actually remarkably similar. (And I think if you asked either director, we’d probably admit that these single-take live recordings do not necessarily reflect our absolute ideal performances). The fact that we independently arrived at near identical readings (in terms of rhythm) leads me to believe that a practical method for performing chant according to the 10th century practice can be taught with as much consistency as the wide-spread Solesmes method. It is not, after all, rocket science: