E. Cobham Brewer 18101897. Dictionary of Phrase and Fable. 1898.
The two-headed eagle. Austria, Prussia (representing Germany), and Russia have two-headed eagles, one facing to the right and the other to the left. The one facing to the west indicates direct succession from Charlemagne, crowned the sixty-ninth emperor of the Romans from Augustus. In Russia it was Ivan Basilovitz who first assumed the two-headed eagle, when, in 1472, he married Sophia, daughter of Thomas Palæologus, and niece of Constantine XIV., the last Emperor of Byzantium. The two heads symbolise the Eastern or Byzantine Empire and the Western or Roman Empire.