For several years, DC Entertainment has been trying to build shared universes in movies and on television. The current efforts now include four TV series and three films (with at least two more on the way). Since DC and Warner Bros. are both Time Warner entities, you'd think it would be easy for their various superhero characters to travel freely among series, and even back and forth between the big and small screens -- but that hasn't been the case. Instead, last season's "The Flash"/"Supergirl" team-up was significant, because their two universes were separated not just by a cosmic vibrational frequency, but by the wall between television networks.

As a result, "Supergirl's" subsequent move from CBS to The CW sparked immediate discussions about a four-way crossover involving "Flash," "Arrow," and "Legends of Tomorrow." Moreover, "Supergirl's" producers just announced that Superman would be appearing in the first two episodes of the show's upcoming season -- played by Tyler Hoechlin, and not "Batman v Superman's" Henry Cavill. Both stories raise questions about the strengths of these various barriers, and whether they might allow more audacious team-ups. Today we'll see what rules might govern these crossovers, and what they might permit.


THE LANDSCAPE



In the past, DC and Warners didn't have much trouble putting out two different adaptations of the same characters at the same time. The Lynda Carter "Wonder Woman" TV series and the Christopher Reeve "Superman" movies ran alongside the animated "Super Friends." Similarly, Brandon Routh starred in 2006's "Superman Returns" while Tom Welling was still going strong on "Smallville"; and the "Batman: Brave and the Bold" animated series contrasted deeply with Christopher Nolan's last couple of Batman movies.

If 2011's "Green Lantern" movie had done better, it might have jump-started its own cinematic universe. Writers Greg Berlanti, Marc Guggenheim and Michael Green turned in a script for "The Flash" which had Ryan Reynolds' Hal Jordan meet Barry Allen in a post-credits sequence.

"GL's" loss turned out to be good for DC's TV fortunes, as Berlanti and Guggenheim joined Andrew Kreisberg on "Arrow"; and the rest is history.

Besides "Flash" and "Legends of Tomorrow," the Arrowverse also includes NBC's "Constantine" and "Supergirl's" parallel Earth. Nevertheless, the Arrowverse stands apart from the other two corners of DC's live-action presence, FOX's "Gotham" and the line of films which started with 2013's "Man of Steel." Looking at the various characters and how they've been adapted for each project, we can start placing them in a few distinct categories.

THE DIVISIONS



1. Movie-Only: These characters are (or will be) starring in their own feature films, so presumably they don't need any "competition," or duplication of stories or other efforts, from any TV counterparts. This group includes Wonder Woman, Batman, Aquaman, Cyborg, Green Lantern, Shazam, Harley Quinn and the Joker.

2. Exact Counterparts: Essentially these are different versions of the same character, at the same point in his/her life, just played by different actors for different projects. Most prominent among these is the Flash, played on TV by Grant Gustin and in the movies by Ezra Miller. Depending on how "Supergirl" treats Superman, and in fact how we define these terms, we might put the Man of Steel in this category as well.

3. Non-Overlapping Counterparts: As above, these are different versions of the same character which are fairly contemporaneous but appear in different media at different times. The most obvious examples are Amanda Waller, Deadshot, Katana and Captain Boomerang, who each appeared on "Arrow" and who will all appear in the upcoming "Suicide Squad" movie. Because the "Arrow" versions are all dead, the two sets probably won't overlap. Other non-overlapping counterparts may include Ra's al-Ghul (of the Christopher Nolan Batman movies and "Arrow") and the bulk of "Gotham's" villains.

4. Inexact Counterparts: These are different versions of the same character, at different points in his/her life, appearing in different media at the same time. Examples include Alfred Pennyworth and James Gordon, both of "Gotham" and the DC Films Universe. We'll talk more about young Bruce Wayne.

5. TV-Only: These characters star in their own series and probably don't need competition from big-budget blockbuster versions. They include Green Arrow, Supergirl, Martian Manhunter, Rip Hunter, Firestorm and the Atom.

THE RULES (OR SOMETHING LIKE THEM)



As near as I can figure, there seem to be two basic rules. First, a character who has been promised to the movies is virtually forbidden from appearing on television; and the reverse may also be true. Second, a character who is not "alive" in one medium can appear on another.

The first rule would seem to prevent any "exact counterparts" as described above. Thus, there can be no TV versions of Batman, Wonder Woman, Aquaman, Cyborg or any Green Lantern. The last is noteworthy in light of "Arrow" co-showrunner Marc Guggenheim's remarks about Golden Age GL Alan Scott. Guggenheim explained that because Alan is "attached to the Green Lantern franchise, which DC obviously has plans for," he can't appear as part of the Justice Society when they show up next season on "Legends of Tomorrow."

Since Alan isn't a member of the Green Lantern Corps, I take it that any restrictions on the big names also extend to affiliated characters. If DC is planning a "Green Lantern Corps" movie for 2020, Alan Scott can't be on TV in 2017. Given the amount of time it takes to put together a GL Corps movie (or, for that matter, to put a single Green Lantern on-screen for a "Justice League" movie), this probably makes a certain amount of sense. Still, it makes me wonder why a Green Lantern Corps movie needs Alan Scott. Arguably it needs Alan a lot less than Ezra Miller's "Flash" movie might need Jay Garrick -- and we've already seen two versions of Jay on this past season of "The Flash."

Speaking of which, not only does the Flash flout this first rule, but so may many of his Rogues Gallery. In two seasons of "The Flash" we've seen Barry Allen fight Captain Cold, Captain Boomerang, Heat Wave, Golden Glider, Gorilla Grodd, two Weather Wizards, two Tricksters, Pied Piper, Rainbow Raider, the Reverse-Flash, Zoom, Vandal Savage and the Turtle. By my reckoning that leaves only Mirror Master, Abra Kadabra and Dr. Alchemy/Mr. Element as classic Rogues with no TV counterparts. (Regardless of medium, I still want Will Arnett to play Abra.) We might also expect other key "Flash" supporting characters to appear in the Ezra Miller movie, including Iris West and Wally West, both of whom appear regularly on TV. Thus, the Flash's TV and film adaptations seem bent on being an ongoing collective exception to these rules.

EASTER EGGS AND OTHER TREATS



"Supergirl" walked right up to a real live Superman appearance without ever showing him in full. He has been a silhouette in the series' credit sequence, a briefly-glimpsed form in a couple of episodes, and (most infamously) a pair of blue-clad, red-booted legs lying unconscious in a hospital bed. The Girl of Steel's series has also mentioned (but never shown) Metropolis, Perry White and Lois Lane. Likewise, Rip Hunter alluded to the deaths of "men of steel and dark knights" when explaining his mission to stop Vandal Savage; and the first episode of "The Flash" mentioned the upcoming merger of Wayne Tech and Queen Incorporated.

Accordingly, while assorted movie-only elements might show up on TV, I suspect that the list of characters off-limits to television is fairly small. Characters who are either unclaimed or associated with projects in limbo may also find their way to TV. A number of magic-based DC characters were probably going to be part of "Constantine" before it was cancelled; so perhaps Doctor Fate, the Spectre, and/or Zatanna may yet show up in the Arrowverse. Likewise, Nightwing, Starfire, Raven and other characters could be eligible for either TV or films if TNT's "Titans" series never makes it out of development.

Regardless, over the past four years the Arrowverse shows alone have produced seven-and-a-half seasons' worth of television and adapted dozens of the comics' characters. Although the movies are doing a lot of team-building, they simply don't have the same capacity to manage large casts and extended story arcs. The TV series are also undoubtedly more free to choose their own directions, and can be more esoteric in their choices (Jonah Hex, Gorilla City, the Legion of Super-Heroes, the Justice Society). In that light the movies are dealing with a lot fewer potential conflicts, leaving the TV series with plenty of options.

Even after going through all of this I am still not sure if there are any hard-and-fast rules about who gets to appear where. I do think there are some practical guidelines governing these adaptations, centered around each production's needs and ability to "share." Under what I theorize is Rule 2, Superman may be on "Supergirl" because his movie counterpart is mostly dead. He might also be available for TV because "Supergirl's" producers have something very specific in mind that doesn't conflict with the movie version's portrayal.

Either way, we're reminded just how big these productions are, and how much they must coordinate with each other, in order to satisfy fans, entertain the public and of course make money. More variety also means a greater capacity to pick and choose favorites.

Still, consider this: right now, in June of 2016, we're already starting to debate whether 2018's "Flash" movie could improve on two seasons of the "Flash" TV show; whether August's "Suicide Squad" will make "Arrow" viewers forget about that show's version; and how Superman's two episodes of "Supergirl" will compare to the character's most recent movie adventures. For fans of DC's characters, those are good problems to have.

What do you think about the DC TV/movie divide? Are there characters you want to appear only on one medium or another, or the more the merrier?