Parallel worlds exist in parallel, alternate universes where, although there are a great many similarities to our known universe, as in the case of Earth-1 with our Superman and the Superman of Earth-2.
This profound cosmological discovery was first made on Earth by The Flash when he inadvertently crossed over to Earth 2 and met his childhood comic book hero, Flash of Earth-2 (The Flash No. 123, Sep 1961: "Flash of Two Worlds!").
Superboy on at least three occasions has come into contact with alternate versions of himself and people he knows.
The first of these encounters occurs when, during a mission in space, an alternate Boy of Steel (who can be differentiated from Superboy by the fact that the colors of his chest insignia are reversed) is hurled from his universe to Earth by the force of an exploding warhead containing Red Kryptonite. Upon his arrival in Smallville, the effect of the Red Kryptonite twists his personality and compels him to ruin the reputation of a good friend, which he attempts to do to Police Chief Parker. When Superboy and Krypto arrive to stop this "false" Superboy, the effects of the Red Kryptonite begin to wear off, and he explains his actions before fading back to his home world (SB No. 116/3, Oct 1964: "The Ordeal of Chief Parker").
The second of these encounters occurs when Superboy is hurled to a different reality by an alien sun when it goes nova. Upon his arrival in the parallel (and mis-spelled) "Smallvile", Superboy meets counterparts of five members of the Legion of Super-Heroes: Brainiac 5, Chameleon Boy, Element Lad, Invisible Kid, and Ultra Boy - who in this universe are evil. During this encounter, the five delinquents attempt to expose Superboy's secret identity as Clark Kent. Superboy, after realizing what they are up to and that he is in fact on a parallel Earth, is able to contact his counterpart, who has been away on a mission. The two Superboys assist each other in thwarting the criminals' efforts and, after apprehending them, return the villainous Legionnaires to their own time (SB No. 117, Dec 1964: "Superboy and the 5 Legion Traitors").
The third of these encounters occurs when Superboy is attempting to avert a tornado disaster in Smallville. The Boy of Steel inadvertently moves fast enough to not only travel backwards through the time stream, but also cross the dimensional barrier that separates Earth-1 and Earth-2. Traveling to the alternate version of his hometown, Superboy encounters his counterpart from this world, who at this time is intent upon using his fledgling super abilities to join a traveling circus. Superboy, after revealing himself to the other Clark Kent, begins training the young farm boy in the use of his super powers. Still intent upon joining the circus, young Clark becomes an acrobatic attraction known as the Masked Wonder, while Superboy attempts to return home. After a brief stint in the circus, young Clark realizes that he is destined for greater things, and returns to his home in Smallville. Once more encountering the stranded Superboy, young Clark uses his super-speed to help create a mini-tornado and aid the hero from Earth-1 in returning home. It is this adventure with the hero from Earth-1 that leads young Clark to his fateful decision some years later to adopt the identity of Superman (NSB No. 15/2, Mar 1981: "Superboy Meets Clark Kent!"; NSB No. 16/2, Apr 1981: "The Superboy Training of Clark Kent!")
Some of the parallel-worlds are:
- Earth-X (Freedom Fighters)
- Earth-X (Steelman)