Flag with a white cross on a red field, supposed to represent an early form of the Reichssturmfahne or  Imperial War Flag, used alongside the Imperial Banner. It was smaller than the imperial banner, carried before the emperor or his appointed commander in battle. It likely developed in the 12th century during the crusades, and was in use as Imperial War Flag during the 13th and early 14th centuries.
Own work (simple geometric design ineligible for copyright)
User:Madden