We have started singing Mass IV in our Sunday Masses. Like most of the ordinary settings in the chant books of the Roman Rite, this ordinary is soubriqueted with the incipit of the trope traditionally sung to the plainchant Kyrie.
The trope appears in the manner below (click on the image to open the file):
In the Middle Ages, this Kyrie occupied the bread-and-butter niche that Missa de Angelis now inhabits amidst many Traditional communities. Troped Kyries were usually sung alternatim in such a way that each Kyrie line was chanted in full before each trope line. Hence, if tropes were still sung in Masses in the usus antiquior nowadays, instead of nine-fold, we would have an eighteen-fold Kyrie.
The tropes in a Kyrie are often arranged such that the first three address the Father; the second, the Son; and the third, the Holy Ghost. This schema is evident in the tropes of this Kyrie. Here are the first tropes for each of the three sets, from the Codex Calixtinus. And here is a sample rendition of some of the tropes.
O almighty Father, God!