Anachronistic representation of the royal banner of the 12th(?) or 13th to early 15th centuries.
The banner of this period would be square in shape. The heraldic eagle is here shown in early modern style.

The black eagle in a golden field was the ensign of the German kings (as opposed to the emperor) in the 12th and 13th century, but in the 14th century, it was gradually re-interpreted as representing the empire. An early reference to such a banner as "imperial" is made in an account of the burial of Charles IV (d. 1378), as der schwartz adler des richs in einem guldin veld .
Own work
Drawing created by David Liuzzo