Barry Allen's title also introduced a much-imitated plot device into superhero comics when it was revealed that Garrick and Allen existed on fictional parallel worlds. Their powers allowed them to cross the dimensional boundary between worlds, and the men became good friends. "Flash of Two Worlds" ("The Flash" (vol. 1) #123) was the first crossover in which a Golden Age character met a Silver Age character. Soon, there were crossovers between the entire Justice League and the Justice Society; their respective teams began an annual get-together which endured from the early 1960s until the mid-1980s.