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The Trans-Mississippi was the geographic area west of the Mississippi River during the 19th century, containing the states of Arkansas, Louisiana, Missouri and Texas, and the Indian Territory (now Oklahoma). The term was especially used by the Confederate States of America as the designation for the theater of operations west of the Mississippi. The geographical term is generally used today only in matters relating to the study of the American Civil War.
However, the term Trans-Mississippi was also used at times to refer to any land "across the Mississippi" or the entire western two-thirds of the United States. In 1898, a Trans-Mississippi Exposition was held at Kountze Park in Omaha, Nebraska. The postage stamps of the Trans-Mississippi Exposition Issue are considered some of the most beautiful stamps ever issued by the US, and a complete set of the "Trans-Miss" is highly prized. In 1998, a set of stamps using designs derived from the original issue was issued to commemorate its 100th anniversary.