The topic of rounds deserves more attention from Mudcat than it seems to get. Some of us tend to believe that vocal folk music in English is characterized by lack of musical sophistication in favour of the lyrics; many English rounds are excellent counter-examples. Folk-revivalists should therefore not exaggerate their self-imposed limitations on musical quality.

The "Sumer" manuscript is rightly regarded as an icon of European music, and a token of England's share. Ironically, it is in fact an "accidentally" surfaced tip of an iceberg. Musicologists who have read the texts of medieval theorists insist that thinking in "chords" had not been invented yet, but I guess that it was well known and frequently practised in folk music, and the monks were actively rejecting it for church music. The same may be true for the imitatio principle, which only later, in the Renaissance period, became the state of the art. Afterwards it almost changed sides to become a characteristic of church music. Chord harmonics had become universal.

Popular (= folkloristic) round singing and composing went on as ever. Many famous and "serious" composers contributed, from many countries. Those of us who refuse to sing rounds for being either not "folkish" or not serious enough, miss out on a lot of dignified and instructive fun. (Of course, like in other forms of art, not everything that is old or appears in print is also good.)

The Wikipedia article about rounds is very poor; a bit more information can be found at Canon - the broader international term for the genre.