Edit to add - pretty sure the first big numbering came in one of his Captain Britain stories & 616 was the number he gave to the main Marvel universe.

I try not to take notice, but in a post (on another forum) years ago, someone said that the Spider-Man/Transformers crossover took place on Earth-634,756,787.*

And my thought was, I loved seeing the web-slinger and the Autobots work together in - what was it? - 1986 or 1987. At no time did I consider what earth it was on; at no point did I care; and at no point did I think it even needed to take place on another earth. A total non-issue.

All I knew was that one of my favourite Marvel superheroes was teaming with a bunch of cool robots. Why wouldn't they? Earth is earth.

Why ruin the fun with designating numbers for different earths?

*A totally made-up number as I can't recall the real one. The truth is, I don't care.

I get the "joke" but I wish he hadn't done that and I wish it hadn't "stuck."

That said, current Marvel editors are banning references to "Earth-616" within Marvel titles. The mainstream Marvel Earth is "Prime Earth." I guess if you're going to have a multiverse that's the best that can be done. 

Those early crossovers enhanced my enjoyment of earlier adventures. When I read the first Superman/Spider-Man crossover, I thought about the Ditko/Lee Spider-Man tales - and how, as the web-slinger was starting his career, the Man of Steel was having adventures a few hundred miles away.

Later on, I thought it was bullshit when publications said that this and that took place on this earth or that earth; and that the first Superman/Spider-Man crossover took place on another earth; oh, and the Spider-Man/Transformers took place on a complete different earth.

How many earths did the DC or Marvel universes need?

There was only Earth-1 and 2 from DC when I left comics.

I learned of all these other Earths just a couple of years ago. Cartoon Earth, TV Earth, Movie Earth, etc. etc....

Do these really need a designation? Can we all not tell what's a comic book from a movie?

Within the Captain Britain storyline, it made sense that the Captain Britain Corps would need some terminology to differentiate between the various dimensions. It's when it bleeds outside that storyline to everything else that it becomes ridiculous. 

So, 8013, 80-1980. 13-? .... I give up.

Roy Thomas and Mark Gruenwald were both fans of the highest order prior to their employment at Marvel Comics. DC had theirs as well, but Roy and Mark specifically went out of their way to have "fun" with the reality of the comic book universes to which they now found themselves contributing. Gruenwald make his mark on the fan world with a highly-regarded fanzine he edited called, "Omniverse" in which he specifically posited theories as to what was "real" in comics and on which Earth events must have taken place if you consider the implications of A. and contrast them with what was shown to be true of B. and so on and so forth.

Thomas had already paid tribute to DC's parallel Earths by creating Earths-S and A, the homes of the Squadron Supreme and the Thing who was Reed Richards instead of Ben Grimm. Upon reading Gruenwald's 'zine, he realized he could take the sci-fi trope of divergent timelines and make a comic from it; a comic in which familiar events from past stories happened differently, creating wildly different results. 

In the intro to the first issue, Thomas had the Watcher chronicle the parallel realities that Marvel was already home to; Killraven's future, Deathlok's timeline, Thomas' own parallel Earths...

And the idea that the Superman/Spider-Man team-up took place on a parallel world. The tabloid came out in 1976 and by 1977 Thomas had effectively removed it from strict Marvel "continuity" by explaining it away as a separate Earth. After all, Marvel was home to parallel worlds no less so than DC. Thomas' own books showed that had been since Earth-S appeared in 1971.*

The numbering system introduced in Captain Britain was put in place in-story by an entity calling himself Merlyn. Since this version of Merlyn apparently lied about his authority as an Omniversal Guardian, among other things, there was never any reason for Marvel in general to accept his numbering as genuine, except, of course, that the fans ate it up, and the more fannish writers and editors who wished to embroider upon it endlessly loved it...

It was these later "contributors" who began numbering the Marvel Earths in the thousands, tens of thousands, and hundreds of thousands, poring over every possible iteration of every character to establish this one on this world, and that on that one... Just as DC's Earths were never Infinite before the mini-series calling them that appeared, Marvel's were never intended to become the vast network of obsessively codified silliness they soon did. Once you start something rolling unfortunately, you sometimes find there are hundreds running along in step behind you who all want to play the game too... regardless of whether they're any good at it or not.

* I don't have the issue at hand, but parallel worlds may have been mentioned earlier in Avengers #70 (1969) when Thomas explains that the Squadron Sinister were created in imitation of the heroes of another world someplace else (thereby hinting *wink, wink* it was the JLA, but later literalizing their inspiration as the Squadron Supreme in 1971.) I'd have to check to be sure.

Gerber had long since lost creative control over the Duck and so Howard was being written by Bill Mantlo in what many fans would argue was a distinctively different style, both in his own magazine and in the Marvel Universe proper, as well as Crazy...

Now Howard, whichever version you pick, comes from Duckworld, itself a parallel version of own world...

We can thereby infer that Gerber's Duck and Mantlo's Duck are two separate incarnations of Howard, and that each then hails from his own version of Duckworld, right? After all, there are matters of fact as well as temperament that separate the two. That would mean that stories featuring Mantlo's must, perforce, take place on a world separate from the Marvel's Mainstream Earth, but yet remain extremely close to it in all other respects, enough so that a cursory review of the story in question wouldn't reveal its separate nature... Marvel Team-Up #96, for instance, as well as Howard's entire magazine run and his appearances in Crazy... 

... Where he appeared side-by-side with the Weems' Hulk... If we assume Weems exists on HIS own Earth, one different from the 616-Prime one we know, then the times he's shown next to Howard must take place on yet ANOTHER parallel world which somehow incorporates the two...

That gives us how many new Earths we can make up numbers for now? Gerber-Earth, Mantlo-Earth, Gerber-Duckworld, Mantlo-Duckworld, Weems-Earth, Weems-HTD-Earth, Weems-HTD-Duckworld... We have at least SEVEN new Earths we can codify and number now to our hearts' content. If we assume Gerber-Earth and Earth-616 are the same, then we have to allow that it and Gerber-Duckworld, a.k.a. Earth 791,021, have already been accounted for. 

We can have the fun, however, of numbering Mantlo-Earth whatever we want... Earth-5,001,001,000, for instance (D-500, U- Written after ensures only that the literal is unsigned so its value here is null, C-100, K- Colloquially means 1,000.) Fun, right?! Hah! Mantlo-Duckworld will just be one number more than actual Duckworld, Earth- 791,022... Weems-Earth can be... let's see... T-Numbers most commonly refer to lens apertures in photography or cyclone force winds..., not much help there, so we won't try to spell "Teen"... How about simply Earth-Seven as a sideways nod of respect to Marie Severin? For the combination Earth, we'll add Earth-7 to Earth-791,022 to come up with... Earth-791,029! Wow, what a blast this is! Are you all having as much fun as I am?

Now, let's look at those Jim Owsley-written Crazy pages in which the lettering to classic stories is redone... I mean, those WERE published by Marvel, right? They have to take place in their own universes too, don't they? Let's begin...

Or not.