Page "Fourth Council of Constantinople (Eastern Orthodox)" Paragraph 3

After the death of Ignatius in 877, Photius mounted the See of Constantinople for a second time.

Since this act was explicitely forbidden by the former Council, another Council was convened in 879.

That council, held at Constantinople, comprising the representatives of all the five patriarchates, including that of Rome ( all in all 383 bishops ), reinstated Photius as Patriarch.