Columbia subject headings:
Harvard subject headings:
Education of princes.
Kings and rulers--Conduct of life.
Kings and rulers--Duties.
Photius I, Saint, Patriarch of Constantinople, approximately 820-approximately 891. Correspondence.
Princeton subject headings:
Boris I, Khan of Bulgaria, -907.
Education of princes.
Facsimiles.
Kings and rulers--Conduct of life.
Kings and rulers--Duties.