It encompasses Sredna Gora, the Upper Thracian Plain, and 90% of the Rhodopes.
The highest temperature recorded in Bulgaria occurred here: it was 45.2 °C (113.4 °F; 318.3 K) at Sadovo in 1916.
Notable cities include Plovdiv, Burgas, Stara Zagora, Sliven, Haskovo, Yambol, Pazardzhik, Asenovgrad, Kardzhali, Dimitrovgrad, Kazanlak and Smolyan.
The Ottoman Empire created the autonomous province of the newly-named Eastern Rumelia in Northern Thrace in 1878.
The population's ethnic composition in the Bulgarian provinces of Burgas, Haskovo, Kardzhali, Pazardzhik, Plovdiv, Sliven, Smolyan, Stara Zagora and Yambol.