Nicene and Post-Nicene Fathers, Ser. II, Vol. XIV:
The Sixth Ecumenical Council.  The Third Council of Constantinople.: Historical Introduction.

p. 326 Historical Introduction.

The Sixth Ecumenical Council met on November 7, 680, for its first session, and ended its meetings, which are said to have been eighteen in number, on September 16th of the next year.  The number of bishops present was under three hundred and the minutes of the last session have only 174 signatures attached to them.

When the Emperor first summoned the council he had no intention that it should be ecumenical.  From the Sacras it appears that he had summoned all the Metropolitans and bishops of the jurisdiction of Constantinople, and had also informed the Archbishop of Antioch that he might send Metropolitans and bishops.  A long time before he had written to Pope Agatho on the subject.

When the synod assembled however, it assumed at its first session the title “Ecumenical,” and all the five patriarchs were represented, Alexandria and Jerusalem having sent deputies although they were at the time in the hands of the infidel.

In this Council the Emperor presided in person surrounded by high court officials.  On his right sat the Patriarchs of Constantinople and Antioch and next to them the representative of the Patriarch of Alexandria.  On the Emperor’s left were seated the representatives of the Pope.  In the midst were placed, as usual, the Holy Gospels.  After the eleventh session however the Emperor was no longer able to be present, but returned and presided at the closing meeting.

The sessions of the council were held in the domed hall (or possibly chapel) in the imperial palace; which, the Acts tell us, was called Trullo (ἐν τῳ σεκρέτω τοῦ θείου παλατίου, τῳ οὕτω λεγομένῳ Τρόυλλῳ).

It may be interesting to remark that the Sacras sent to the bishops of Rome and Constantinople are addressed, the one to “The Most holy and Blessed Archbishop of Old Rome and Ecumenical Pope,” and the other to “The Most holy and Blessed Archbishop of Constantinople and Ecumenical Patriarch.”  Some of the titles given themselves by the signers of the “Prosphoneticus” are interesting—“George, an humble presbyter of the holy Roman Church, and holding the place of the most blessed Agatho, ecumenical Pope of the City of Rome…,” “John, an humble deacon of the holy Roman Church and holding the place of the most blessed Agatho, and ecumenical Pope of the City of Rome…,” “George, by the mercy of God bishop of Constantinople which is New Rome,” “Peter a presbyter and holding the place of the Apostolic See of the great city Alexandria…,” “George, an humble presbyter of the Holy Resurrection of Christ our God, and holding the place of Theodore the presbyter, beloved of God, who holds the place of the Apostolic See of Jerusalem…,” “John, by the mercy of God bishop of the City of Thessalonica, and legate of the Apostolic See of Rome,” “John, the unworthy bishop of Portus, legate of the whole Council of the holy Apostolic See of Rome,” “Stephen, by the mercy of God, bishop of Corinth, and legate of the Apostolic See of Old Rome.”