Alternative versions of Superman establish a new continuity for DC, affecting not only Superman, but all of the DC Comics characters. The versions of Superman from this time period are traditionally divided into three main periods. The first version of Superman began being published by DC Comics in 1938 appearing in "Action Comics" #1. To explain discrepancies in the aging of Superman across several decades, his earliest stories were retroactively portrayed as having taken place on an alternative world called Earth-Two. These stories take place from 1938 until the late 1950s, although the exact dividing line is unclear and some stories happened identically to
Superman Universe, which is a shared universe of superhero characters owned by DC Comics, and consequently he frequently appears in stories alongside the likes of Batman, Wonder Woman, and others. Superman has sold more comic books over his lifetime than any other American superhero character. Exact sales figures for the early decades of Superman comic books are hard to find because, like most publishers at the time, DC Comics concealed this data to deny competitors, but sales of "Action Comics" and "Superman" probably peaked in the mid-1940s and thereafter steadily declined as part of a general trend in comic book sales.