The concept of a shared universe in comics involves writers and editors, together with artists who together create a series of titles where events in one book would have repercussions in another title and serialized stories would show characters grow and change. This idea was strongly developed in the Marvel Universe in the early 1960s, and seen also in other publishers in recent years.
Headline characters in one title would make cameo or guest appearances in other books. The leading heroes of the DC Universe were originally (in the 1940s) published in a team book known as the Justice Society of America. In the 1960s, this concept was revamped in the book named the Justice League of America. The DC Universe typically has its comic books set in fictional cities, such as the twin cities of Gotham City and Metropolis which were fictional analogues to New York city). These cities were portrayed in as fictional archetypes, with Gotham City embodying the negative aspects of life in a large city, and Metropolis having the positive aspects. The presence of superhumans affected the cities, but the general history of the fictional United States was similar to the real one.
Over the years as the number of titles published increased and the volume of past stories accumulated it became increasingly difficult to maintain internal consistency. In order to continue publishing stories of its most popular characters, maintaining the status quo became necessary. Retcons were used as a way to explain apparent inconsistencies in stories written. Change and growth for characters was replaced with the illusion of change.
DC Comics, in contrast to Marvel Comics, drastically rebooted their continuity several times. DC Comics had used a sophisticated alternate world theory to explain the multiple occurrences of their major characters written at various times in history. In recent years, stories involving this explanation were increasingly inaccessible for neophyte readers. Since the number of young readers of comics has been steadily decreased, efforts such as Zero Hour (the second reboot) were initiated to simplify the storylines. Later, a sequel series to the mini series, Kingdom Come called The Kingdom reintroduced a variant of the old multiverse concept called The Hypertime which essentially allows for the parallel worlds again.