 "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!"