as some published under its imprints Vertigo and Young Animal) take place within a shared universe called the DC Universe (DCU) allowing plot elements, characters, and settings to cross over with each other. The concept of the DCU has provided DC's writers some challenges in maintaining continuity, due to conflicting events within different comics that need to reflect the shared nature of the universe. "The Flash of Two Worlds" from The Flash #123 (September 1961), which featured Barry Allen (the Silver Age Flash) teaming up with Jay Garrick (the Golden Age Flash) was the first DC comic to suggest that the DCU was a part of a multiverse. The DC Multiverse concept was expanded in later years with the DCU having infinite Earths; for example, the Golden Age versions of DC heroes resided on Earth-Two, while DC's Silver Age heroes were from Earth-One. Since "Crisis on Earth-One!" (1963), DC has used the word "Crisis" to describe important crossovers within the DC Multiverse. Over the years, various writers took liberties creating additional parallel Earths as plot devices and to house characters DC had acquired from other companies, making the DC Multiverse a "convoluted mess". DC's comic book sales were also far below those of their competitor Marvel Comics. According to ComicsAlliance journalist Chris Sims, "the [DC] multiverse . . . felt old-fashioned. . . . Marvel, on the other hand, felt contemporary and when you stack them up against each other, there's one difference that sticks out above anything else: Marvel feels unified." During the Bronze Age of Comic Books, writer Marv Wolfman became popular among DC's readers for his work on Weird War Tales and The New Teen Titans. George Pérez, who illustrated The New Teen Titans, also began to rise to prominence in this era. In 1984, Pérez entered into an exclusive contract with DC, which was later extended one year. Although The New Teen Titans was a major success for DC, the company's comic book sales were still below Marvel's. Wolfman began to attribute this to the DC Multiverse, feeling "The Flash of Two Worlds" had created a "nightmare": it was not reader-friendly for new readers to be able to keep track of and writers struggled with the continuity errors it caused. In The New Teen Titans #21 (July 1982), Wolfman introduced a new character: the shadowy, potentially villainous Monitor; this laid the foundation for Crisis on Infinite