Columbia subject headings:
Boris I, Khan of Bulgaria, -907.
Bŭlgarska pravoslavna t︠s︡ŭrkva--History--Sources.
Photius I, Saint, Patriarch of Constantinople, approximately 820-approximately 891. Correspondence.
Schism--Eastern and Western Church--History--Sources.
Harvard subject headings:
Boris I Michael, tsar of Bulgaria, ruled 852-889.
Photius I, Saint, Patriarch of Constantinople, approximately 820-approximately 891.
Princeton subject headings: