By the early 1950s superhero comics weren't selling too well. So with the exception of the big three (Superman, Batman, and Wonder Woman) many DC Comics titles were canceled. Super heroes were replaced by cowboys, monsters, and funny animals. So the likes of Flash, Green Lantern, Hawkman, and others disappeared from the scene.
Nearly a decade later, at the dawn of comics' Silver Age, editor Julius Schwartz began to revive some of the old heroes. New origins, costumes, and secret identities were given to the Flash, Green Lantern, the Atom and others. It was finally decided that it would be pretty cool if the two Flashes could meet. So in an adventure called "Flash of Two Worlds," (in The Flash # 123) the newer Flash (Barry Allen) accidentally traveled to a parallel earth where he met his Golden Age counterpart, Jay Garrick. This was the first Earth-Two story in DC Comics.
Earth-Two was designated home of the Golden Age heroes of the 30s and 40s. These heroes were somewhat older and had begun their careers earlier than their Earth-One counterparts. While the Earth-Two heroes had lived through the World War II era, the Earth-One heroes were born a generation later. While the Golden Age heroes of Earth-Two had banded together as the Justice Society of America, the Silver Age heroes of Earth-One formed the Justice League of America. Team-ups of the two groups became an annual occurrence in the pages of Justice League of America.
But what about the big three (Superman, Batman, and Wonder Woman)? Their books had never gone away, so they had never been revamped with new origins, new costumes, or new secret identities. They had each held membership in both the Justice Society and the Justice League. How could this be since these two teams existed on separate earths?
Readers eventually learned that there were indeed Earth-One and Earth- Two versions of each of the big three. At some point in their respective titles, it was assumed that the Earth-One version of each hero took over the solo adventures of their Earth-Two counterpart. It's a bit difficult to pinpoint in exactly which issues these transitions took place. But it's certain that the Batman stories occurring after Detective Comics # 327 were adventures of the Earth-One Batman. This was the first issue of the "new look" Batman, whose costume had been modified to include the bat insignia against a yellow ellipse.
In the 60s and 70s, readers learned that the Earth-Two Batman, considerably older than his Earth-One counterpart, had retired from costumed crime-fighting. His alter ego, Bruce Wayne, had eventually become Gotham City police commissioner. Wayne had married his reformed nemesis Selina Kyle (a.k.a. Catwoman). They had a daughter who grew up and fought crime as the Huntress. She became a member of the JSA along with a grown-up Robin.
The Clark Kent of Earth-Two had gone from newspaper reporter to editor of the Daily Star. He had married Lois Lane while the romantic tension between their Earth-One counterparts remained intact. While the Earth- One Superman's Kryptonian cousin was Supergirl, on Earth-Two it was Powergirl, a character created in the late 70s.
So the multiverse was born. And if two earths were good, more would be better. When DC bought the rights to Fawcett characters including the Captain Marvel family, those heroes resided on Earth S ("S" for "Shazam"). When rights to Quality characters were purchased they became residents of Earth X, a strange world on which the Nazis had won World War II.
The many earths of DC Comics were alike in many ways, but with variations. As mentioned, the Earth-Two heroes were a generation older than their Earth-One counterparts. The Earth-Two heroes seemed to age in real time. The multiple earths gave a ready explanation for why the big three had never appeared to age in comics. It's because the earlier issues were about the Earth-Two heroes, while the later issues chronicled adventures of the younger, Earth-One versions of Superman, Batman, and Wonder Woman.
This post grew out of my efforts to articulate these concepts to the uninitiated. So how am I doing? If anyone has questions, or anything to add, I'd love to read your comments. I'll have more multiverse history at a later date.