In England the gymel, or cantus gemellus (twin song), flourished at an even earlier date.    From Wordnik.com.  [The Catholic Encyclopedia, Volume 4: Clandestinity-Diocesan Chancery] Reference

While, up to the twelfth century, the concept of harmony was restricted on the Continent to the simultaneous sounding of the intervals of the fourth below the melodic note with its octave above, in the British Isles — in their gymel (cantus gemellus) — they were using also the interval of the third both below and above the melodic note, and, by transposing the third below an octave higher, they created the so-called falso-bordone, faux-bourdon, false bass, or three-part harmony (inverted triad), as we know it to-day.    From Wordnik.com.  [The Catholic Encyclopedia, Volume 7: Gregory XII-Infallability] Reference