Jost de Negker, Das hailig römisch reich mit sampt seinen gelidern (The Holy Roman Empire and all its Members), 1510

The double eagle became established as a symbol of the Holy Roman Empire in the 15th century. A particularly popular version was the "quaternion eagle" depicting the estates of the Empire on its remiges, with the eagle standing for the Empire as a whole. The system of imperial quaternions emerged in the early 15th century a way of visualising the hierarchy of estates within the Empire. The estates are grouped in fours of equal rank, the so-called "quatuorvirates".

Jost de Negker (1485–1544), Das hailig römisch reich mit sampt seinen gelidern (The Holy Roman Empire and all its Members), wood cut, 1510; source: Wikimedia Commons, https://commons.wikimedia.org/wiki/File:Quaterionenadler_David_de_Negker.jpg, public domain.