Medieval literature
In the Middle Ages, only the original, and undamaged sealing wax impressions, were accepted as valid. Forged documents were common, and a well-organized archive of treaty documents that could serve as a reference did not exist. Medieval literature mentioned a link between the Frisian eagle as a heraldic charge in coats of arms and the Karelsprivilege. This is unclear because in the time of Charlemagne there were no family or regional coat of arms existing, but it could have been used as a banner in that period.