On October 15th, 2014, /u/switchfade asked the following on /r/DCcomics:
Can someone help me? I’m trying to read about earth D! I found the concept amazing but I have no idea how to find out more
DC history nerd here! I think I can probably help :)
First we need to go way way back to the 60’s. The decision to explain how the the 1940’s Flash (Jay Garrick, the guy with the metal hat) and the new Flash (Barry Allen) could both exist was to say they existed on parrallel Earths, and the two finally met in the seminal story Flash of Two Worlds (1961). This lead to the establishment of Earth-One, where the modern characters lived, and Earth-Two, where Jay Garrick and the Justice Society hung out. The JLA and the JSA the had annual adventures where they’d team up for, basically, the next two decades.