The numeric designations of these alternate universes have been confirmed by Marvel Comics throughout the years and compiled in 2005's "Official Handbook of the Marvel Universe: Alternate Universes", and in Marvel publications since the release of the "Handbook". The prevalent method of labeling an unnamed universe is to derive numbers in some way from the publication date of the relevant issue featuring its first appearance. This is, in turn, based on the mistaken belief that "Earth-616" derived its number from the publication date of "The Fantastic Four" #1 (Nov. 1961), although the origin of this term in particular has been under debate. One active theory is that it came from publications by Marvel UK from Saturnyne mentioning that Captain Britain was from Earth-616.