The Onon is a river in Mongolia and Russia, 1,032 kilometers (641 mi) long.  It originates at the eastern slope of the Khentii Mountains, then for 298 km it flows within Mongolia before entering the Khentei-Daur Highlands region of the Russian Far East.  In its lower reaches it flows between the Mogoytuy (Могойтуйский хребет) and the Borshchovochny (Борщовочный хребет) mountain ranges. It joins with the Ingoda to form the Shilka.  Wikipedia can be such fun this way.  Unused card from Mongol Post.  Grade: 1