This is footnote to my recent (second) series on Concelebration; unlike the earlier posts, in which I shared facts, in this one I now advance a hypothesis.
"Pray Brethren that my sacrifice and yours ..."
We find the roots of this formula, which precedes the Prayer Over The Offerings, in Carolingian Gaul, in a rubric which goes: "Then indeed the Priest to [or with?] right hand and left asks of the other priests that they pray for him".
I am suggesting that originally the Orate Fratres was a formula addressed to concelebrants; although, of course, through being used by celebrants who had no concelebrants around them, it soon came to be thought of as addressed to the assistant clergy in the sanctuary and to the congregation.
The strength of my new theory is that it makes sense of the concept of "my sacrifice and yours". I have long been puzzled by the assumption we have all made that a formula which entered the Mass in the Carolingian period should seem to want so explicitly to refer to the People as offerers of the Sacrifice. Yes, I know that in a sense they certainly are, but that was a period in which emphasis was laid more and more strongly on the idea that the Priest sacrifices for the people (so that the phrase "for whom we offer unto thee" entered the Memento).