Columbia subject headings:
Bulgaria--History--Congresses.
Bulgaria--Relations--Poland--Congresses.
Bulgarian philology--Congresses.
Poland--Relations--Bulgaria--Congresses.
Harvard subject headings:
Bulgaria--History--Congresses.
Bulgaria--Relations--Poland--Congresses.
Bulgarian philology--Congresses.
Poland--Relations--Bulgaria--Congresses.
Princeton subject headings:
Bulgaria--History.