The founders of my country (Republic of Macedonia), initially choose the ethnic boundary in order to establish the country. Unfortunately, that turned out to not be possible because of occupation from three other countries, and on top of that there was genocide and forced movement of people.
When the country finally came in a situation to gain independence, the boundaries were established in accordance to the physical features, with the border predominantly being along the highest points of the mountains. Also, the present day boundary is the ''boundary'' of one of the three regional parts of Macedonia, the Vardar part of it, so it is a mixture.