Columbia subject headings:
Bulgaria--Foreign relations--1878-1944--Sources.
Bulgaria--Relations--Poland--Sources.
Poland--Relations--Bulgaria--Sources.
Harvard subject headings:
Bulgaria--History--Boris III, 1918-1943--Sources.
Bulgaria--Relations--Poland--Sources.
Poland--History--1918-1945--Sources.
Poland--Relations--Bulgaria--Sources.
Princeton subject headings:
Bulgaria--Foreign relations--1878-1944--Sources.
Bulgaria--Relations--Poland--Sources.
Poland--Relations--Bulgaria--Sources.