Map from "Report of the International Commission to Inquire into the Causes and Conduct of the Balkan Wars" 1914.
"The Serbian-Bulgarian treaty of 13 March 1912 made provision for the partition of Macedonia along the following lines: 'all the territory north of the Sar range' was to go to Serbia; 'all the region east of the Rhodope range and the Struma valley' was to go to Bulgaria. Bulgaria hoped the intervening country should form an 'autonomous Macedonia', but, if this should prove impossible, a new line was to be drawn leaving Kumanovo, Skoplje and Debar to Serbia, and giving Kratovo, Veles, Bitolj (Monastir) and Ohrid to Bulgaria. Serbia undertook to make no claim south of the line; Bulgaria reserved the right to claim territory to the north, in which case Russia was to act as arbitrator. The area of overlapping claims was known as the 'Contested Zone'. "...quote from: Great Britain. Naval Intelligence Division, Geographical Handbook Series: Jugoslavia, Volume II, 1944, p. 114