Columbia subject headings:
Bulgaria--Civilization--Congresses.
Bulgars (Turkic people)--Congresses.
Harvard subject headings:
Bulgaria--Antiquities--Congresses.
Bulgarians--History--Congresses.
Bulgars (Turkic people)--History--Congresses.
Slavs--History--Congresses.
Princeton subject headings:
Bulgaria--Civilization--Congresses.
Bulgars (Turkic people)--Congresses.