Allen, Jay Garrick's world was a work of fiction as it was in the real world. This story not only presented the encounter of two worlds and the existence of the Multiverse for the first time, it also presented key features of the Multiverse: all the universes vibrate at a specific frequency which keeps them separated; these "barriers" could be trespassed by "tuning" to that vibration. Because people could also "tune-in" these worlds in dreams, some people wrote comic books with the stories from those worlds they dreamed, which explained why Barry Allen knew about Jay Garrick as a fictional character. The success of this story led to the first team crossover between the new Justice League of America (JLA) and the Golden Age JSA, in the stories "Crisis on Earth-One" (Justice League of America #21) and "Crisis on Earth-Two" (Justice League of America #22). This story arc started the tradition of a yearly crossover between the JLA and the JSA, and established firmly the concept of a Multiverse and the designation of names, Earth-One being the JLA reality and Earth-Two the JSA reality. The success of these crossovers spawned publications telling the further stories of the Golden Age heroes in the present day parting from many of the stories told, thus establishing a more defined continuity for every universe. This concept of parallel Earths with differences in locations, persons and historical events became a very important ingredient within DC Comics' publications. It helped (among other things) to explain continuity errors, to retell and retcon stories, and to incorporate foreign elements that could actively interact with everything else and allow them to have an "existence". Continuity flaws between the established Earth-Two and several stories from the Golden Age were given separate Earths. "Imaginary" stories and some time divergences of Earth-One were given also separate realities (such as Earth-B and Earth-A). In addition to the stories appearing mainly in the pages of JLA that created new Earths, the acquisition of other comic book companies and characters by DC Comics incorporated these new properties as Earths in the Multiverse which would interact several times with the "main" Earths, One and Two; two of the most notable being Earth-S for the Fawcett Comics characters, and Earth-X for the Quality Comics characters. By the 1970s, everything that was published or related officially to DC Comics' titles could become part of the Multiverse, although much