Two corps of the Union Army were called XX Corps during the American Civil War. Though both served in the Union Army of the Cumberland, they were distinct units and should be recognized as such.
General McCook with the staff at Chickamauga under the Corps banner
General McCook's headquarters at Stevenson, Alabama
The Army of the Cumberland was one of the principal Union armies in the Western Theater during the American Civil War. It was originally known as the Army of the Ohio.
Battle of Missionary Ridge
Maj. Gen. William S. Rosecrans
Maj. Gen. George H. Thomas