A good example of this sort of practice would seem to be the famous 13th century English rota (or "round"), Sumer is icumen in, usually considered as either of "folk" origin, or inspired by "folk" performance practices common in England at the time. As you can see, the manuscript bears both a secular text, in the Old English vernacular, at the top, and, underneath, a completely different liturgical Latin text, as though familiar "popular" music were being used to make it easier for the congregation to relate to Catholic liturgy. Such practices are common in many Christian churches today.
This is the earliest example we have of a notated round or canon and, interestingly, it is presented in a manner that has become standard for such musical structures from then on. Only the basic elements are notated, in this case the principal melody, on the first six staves, and the melody of a second canonic pes, or "ground" on the bottom-most staff. The red cross on the first line indicates where the second voice is to enter, singing the exact same melody from the beginning as the first voice sings the next section. And so on, as other voices enter at the same time interval. In other words, what you are seeing is not a full score, but only the melodic material and the rule (or "canon") that tells the singers what to do. To make the resulting counterpoint clear, I've concocted the following score, representing the first 6 measures (assuming 12/8 time):
Here's a clip from an excellent performance, on the CD "Medieval Songs and Dances," by the group "St. George's Canzona." Note that the bottom two staves contain the "pes," a second canon of two bars that "grounds" the four part canon above it.The excerpt begins with two statements of this pes, before the upper voices enter. By the way, my score is based on the original version, complete with "rule violations" subsequently corrected by someone who overwrote the original. While the "corrected" version is better known, the original interests me more, as will be made clearer in the next post, where I'll have much more to say about this remarkable work.