This ad from 1944 shows almost the entire range at that point. For some reason they forgot Leading Comics (and Picture Stories from the Bible, which might have been more of a deliberate omission), but otherwise these are all the titles that people see as comprising the DC Universe at that time. They were still actually published by two closely linked but separate companies then, but the lines between them were very blurry.
But the idea of a "DC Universe" in which all the stories in these anthology comics took place was entirely unheard-of then. Each superhero or non-superhero supporting-feature clearly existed in its own fictional reality. Superman didn't exist in the world where Zatara the Master Magician had his adventures in the back pages of Action Comics, let alone the other comics with a "Superman DC" logo on the cover but no sign of Superman inside.
But does that mean ALL the superheroes under the DC banner are part of that reality? It's sort of implied, and modern-day comic creators and fans have always taken the view that any comic character published by DC (or the other companies they've purchased over the years) is fair game to include in a "DC Universe" story set in the 1940s or later, but I think we need to be a bit more strict than that.
Let's establish once and for all which heroes can be shown to exist in the shared DC Universe of the Golden Age! We'll just need to set up a few simple ground rules...
1) The Golden Age doesn't really have a defined end point, but there's a pretty clear cut-off in the early fifties when all the superheroes had been cancelled and forgotten due to lack of interest, except the select few (Superman, Batman, Wonder Woman and don't forget Superboy, Aquaman and Green Arrow in Adventure Comics). We only care about comics published during the time before that cut-off.
2) If two superheroes are seen to exist in the same universe in one comic story, then that means ALL the appearances of either character must exist in the one universe! Even when there's another story where Superhero A says that Superhero B is a fictional character! This is a standard rule of comic logic, believe me.
4) The characters have to at least appear in the same story - in various stories people will mention Superman, but it's usually in an ambiguous kind of way. They might be talking about the fictional character in Action Comics. Actually, sometimes superheroes talk about themselves as being fictional characters in DC Comics, but if they're seen to physically interact with each other, they're in the same universe!
So where do we start building the universe? There's only one place - the Justice Society of America!
Quite a big universe already; bigger than any other comics publisher of the 1940s can claim! But can we add more?
Oh yes, quite easily! See, there was another superhero group in DC comics - Leading Comics No. 1 to No. 14 chronicled the adventures of the Seven Soldiers of Victory! All eight of them. It was five heroes and three sidekicks, but one of the sidekicks (the comedy Chinese one) doesn't count as a member.
Superhero interactions like that were really, really rare outside the two super-team comics at DC. Why that one exists is a real mystery, but there are a tiny handful of others that are useful to us too...
Hop wasn't just featured in text stories. His comic series was a mainstay of All-American Comics, and he had his own long-running radio show and a movie serial too! And yes, radio shows would count in this exercise, and so would the movie serials and newspaper strips and anything else - as it happens, there was almost no superhero interaction in them, except Batman showing up in Superman's radio show, but ask any comic reader of the time if those things "exist in a different fictional universe" and they'd look at you like you were some kind of loony.
But if we're counting text stories, do we also have to count editorial pages that seem to imply that superheroes exist in the same universe? It's a bit awkward, but I'm thinking no, we don't.
I'm going to say no. It has to be a proper story, or it just gets silly. And we wouldn't want this whole exercise to be silly, now would we? If we count this kind of thing, we'd need to count the similar poll in the early issues of Sensation Comics, implying that the Black Pirate would like to join the Justice Society too. And he lives in the 16th century. What, are we supposed to believe in some kind of magic time travel? You have to draw the line somewhere.
And that's basically it for crossovers. The ones documented here are the only occasions in golden age DC where the inhabitants of our shared universe bump into each other. But if you're planning to read every in-universe story in the comics, don't forget the spin-offs!
Sometimes a supporting character would be promoted into their own stories. It's probably not worth mentioning that stories in the Superman comic with "Lois Lane" in the title panel still count as part of the shared universe. But it's perhaps less obvious when the Star-Spangled Kid (yes, him again) acquired a new co-star and then just disappeared forever. The last four installments of what used to be his series are simply titled Merry, Girl of 1000 Gimmicks.
Those are all unquestionably part of our integrated Superman DC Comics Golden Age Reality. So if you're wanting to read the entirety of the DC Universe up to, say, the end of 1950, and want to make sure you ignore anything that wasn't definitely part of it as established by comics published before that date, just read anything with one of the 33 titles in bold red font above!
(I mean, you should probably read the other stuff too. It's not Aquaman's fault no other superhero wanted to talk to him in the 1940s. And the funny-animal comics are often better than the superheroes anyway!)
