of it remained largely uncatalogued. The names of the worlds were usually in the format Earth, hyphen, spelled numeral/letter/name. In the case of worlds with numerals, the "rule" of spelling the number was not always followed, even within the pages of the same issue. Crisis on Infinite Earths As the 50th anniversary of DC Comics drew near, major events were proposed for the celebration: an encyclopedia (Who's Who in the DC Universe) and a crossover of the ages, characters, and worlds appearing in DC's comics. As told in the letter section of Crisis on Infinite Earths #1, as the research started in the late 1970s, it became evident that there were many flaws in continuity. The way used to circumvent some of these errors was the "Multiple Earths", which also showed a chaotic nature that brought even more continuity problems that were not easily explained or were simply left unexplained. Examples of this included: 1) the Black Canary of Earth-One being the daughter of the original Black Canary of World War II even though the original Black Canary was a resident of Earth-Two, and 2) the existence of Golden Age comic books on Earth-One and the people not noticing that some of the characters in those comic books existed in "real life". In addition, many universes had multiple alternate timelines, such as Kamandi and the Legion of Super-Heroes both being from Earth-One. Writer Marv Wolfman took this crossover event as an opportunity to reform all the fictional universe of DC Comics to avoid further continuity errors and update the DC characters to modern times. The whole Multiverse is destroyed except for five Earths (the Silver Age's Earth-One, the Golden Age's Earth-Two, Charlton Comics' Earth-Four, Fawcett Comics' Earth-S and Quality Comics' Freedom Fighters' Earth-X). Later, the universe is recreated as one single universe from those five. The crossover event Convergence (2015) officially retconned the events of Crisis after heroes in that series went back in time to prevent the collapse of the Multiverse. Modern Age DC Comics After the conclusion of Crisis on Infinite Earths, the concept of a single Universe containing most elements of the "surviving Earths" was established and heavily enforced to avoid the continuity problems of the Multiverse. However, alternate realities that affected the new DC Universe appeared very quickly. In Superman (vol. 2) #8, a Universe inside the Universe was revealed to have been created to preserve