Natural geographical territorial units of Bulgaria Abbreviations used: B – Black Sea Coast; B1 – Western Stara Planina (Balkan) Mts.; B2 – Middle Stara Planina (Balkan) Mts.; B3 – Eastern Stara Planina (Balkan) Mts.; BN – Northern Black Sea Coast; BS – Southern Black Sea Coast; D – Danubian Plain; DM – Middle Danubian Plain; DW – Western Danubian Plain; E1 – Popovo-Provadiya district; E2 – Loudogorie-Dobroudzha district; P1 – Western Predbalkan; K1 – Rouy Mt.; K2 – Trun Basin (Znepole); K3 – Strazha-Cherna Gora-Rudini Mts.; K4 – Golo Burdo Mt.; K5 – Verila Mts.; K6 – Kraishte; K7 – Zemenska Planina Mt.; K8 – Konyavska Planina Mt.; K9 – Kyustendil Basin; O1 – Osogovska Planina Mts.; O2 – Vlahina Planina Mts.; O3 – Maleshevska Planina Mts.; O4 – Ograzhden Mts.; O5 – Belasitsa Mts.; O6 – Srednostroumska Valley; O61 – Boboshevo-Simitli Valley; O62 – Kroupnik-Sandanski-Petrich Valley; P2 – Middle Predbalkan; P3 – Eastern Predbalkan; R – Rila-Rhodope Massif; R1 – Rila Mts.; R2 – Pirin Mts.; R3 – Slavyanka Mt.; R4 – Sturgach Mt.; R5 – Mesta Valley; RE – Eastern Rhodope Mts.; RO – Osogovo-Belasitsa group; RP – Rila-Pirin group; RR – Rhodope Mts.; RW – Western Rhodope Mts.; S – Stara Planina Range System; S1 – Podbalkan Basins; S2 – Sredna Gora Mts.; S21 – Ihtimanska Sredna Gora Mts.; S211 – Lozenska Planina Mt.; S22 – Sushtinska Sredna Gora Mts.; S23 – Surnena Sredna Gora Mts.; SB – Stara Planina (Balkan) Mts.; SP – Predbalkan; T – Transitional Region; T1 – Sakar-Toundzha district; T11 – Sakar Mt.; T2 – Bakadzhik-Bourgas district; T3 – Strandzha-Dervent district; T31 – Strandzha Mts.; TK – Kraishte-Konyavo district; TL – Thracian Lowland; TS – Srednogorie-Podbalkan subregion; TT – Toundzha-Strandzha subregion; TV – Vitosha district; V1 – Sofia Basin; V2 – Zavalska-Viskyar Mts.; V3 – Lyulin Mt.; V4 – Vitosha Mt.; V5 – Plana Mts.