In the Army of the Potomac, the Cavalry Corps used a swallow-tailed guidon with white crossed sabres centered over two horizontal stripes. The red over blue colors designated General George Armstrong Custer’s headquarters. Custer was the youngest man given the rank of Major General in US military history, but when the US Army was downsized in 1866, Custer, because he lacked seniority and time in rank was temporarily reduced to his last permanent rank of Captain. Because of his outstanding war record, he was soon promoted up to Lt. Colonel and placed in command of the newly formed 7th Cavalry.
This image is a reasonably accurate stylization of the four guidons which General Custer carried during the war; all were red over blue with white crossed sabers, but all were different. Other versions can be seen on the Chart of More Northern Regimental and Unit Flags.