The 1960s saw the introduction of a second Superman, Kal-L. DC Comics had established a multiverse within the fictional universe its characters shared. This allowed characters published in the 1940s to exist alongside their updated counterparts published in the 1960s. This was explained to the reader through the notion that the two groups of characters inhabited parallel Earths. The second Superman was introduced to explain to the reader Superman's membership of both the 1940s superhero team the Justice Society of America and the 1960s superhero team the Justice League of America.