 These months are called "Intercalaris Prior" and "Intercalaris Posterior" in letters of Cicero written at the time; there is no basis for the statement sometimes seen that they were called "Undecimber" and "Duodecimber", terms that arose in the 18th century over a millennium after the Roman Empire's collapse.Their individual lengths are unknown, as is the position of the Nones and Ides within them.Because 46 BC was the last of a series of irregular years, this extra-long year was, and is, referred to as the "last year of confusion".