The western boundary of Romania is with Hungary.
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The Ural Mountains and the Ural River mark the western boundary of Asia.
British territory surrounded the globe. there was no western boundary.
The Mississippi River forms the western boundary of the State of Illinois.
The Rocky Mountains formed the new western boundary of the US after the Louisiana Purchase in the early 19th century.
The eastern boundary of ancient Egypt was typically the Red Sea, although it varied at times. The western boundary was the Western Desert. The northern boundary was the Mediterranean Sea.
In 1802, the Mississippi River formed the western boundary of the United States. The southern boundary extended to the 31st parallel north latitude.
The Colorado River.
The prime meridian at longitude 0° marks the eastern boundary of the Western Hemisphere The corresponding anti-meridian at Longitude 180° marks the western boundary of the Western Hemisphere. (In practical terms you could also consider the international date line to be the boundary - just not the official boundary.)
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