The physics differed drastically in the divergent dimension of Earths One and Two, and among these was the affect on time with living beings. In one universe, a temporally displaced individual could not exist in the same time as the past or future version of themselves. In another universe, an individual could exist as multiple copies of themselves at the same time. However, in that second reality, it was impossible to create a chronal clone of themselves while in the first it was entirely possible.
Two time tyrants discovered these core principals in their native realms and more than once manipulated these quirks in physics to their own amoral agendas. These villains were T.O. Morrow of Earth-One, and Per Degaton of Earth-Two, and both men were persistent foes of the Justice League and Justice Society, respectively.
T.O. Morrow first appeared in Central City, when he spawned three chronoal counterparts of Hal "Green Lantern" Jordan, complete with identical power rings. These three synthetic beings were sent by their creator to steal precious items from the Louve in Paris, the Statue of Liberty in New York City, and London. This terrible trio taunt Green Lantern and his Justice League teammate the Flash, who each pursue one of the fake Lanterns in Paris and New York, and then the pair seemingly teammup in London the Flash determines that his partner is one of the fakes. The real Lantern was defeated in New York City, and after freeing his friend, the Leaguers track down Morrow at his headquarters while the psuedo-Lanterns fade from existence. Although initially tricked by the T.O. Morrow temporal clone, the heroes discover this rouse and corner the real Morrow. Seeing no way out, the villain seemingly falls to his death in an underground pit.
Per Degaon hatched the third of this history-altering schemes when he recruited the Crime Syndicate of Earth-Three to accompany him to Earth-Prime, where the rogues stole nuclear weapons from Cuba. Bringing them back to Earth-Two in 1942, Degaton sought to blackmail the world into accepting his authoritarian rule, which in one future timeline was successful until the early 1980s. However, the Justice League of Earth-One travelled back to that era and teamed up with the All-Star Squadron to oppose him, and soon found their friends from the future appear in the skies above one of Degaton's nuclear detonation demonstrations at sea. These were the 1980 versions of 1940s Justice Society members Green Lantern, Starman and Doctor Fate... along with superheroines Power Girl and Huntress. The three male Society members were the chronal counterparts of Morrow's initial two Green Lanterns and himself (while the 1980s Power Girl, who also existed in the 1940s, was the chronal counterpart of Supergirl's Satan Girl/Nightflame clone from the 1970s).
The third of Morrow's Green Lantern temporal twins had a counterpart in the Conqueror's synthetic Green Lantern, which he used against the original and his Justice Society allies. Years later, T.O. developed chronal clones of Steve Trevor, Mera, Hawkgirl, Jean Loring and a girl name Midge... in order the ensnare Wonder Woman, Aquaman, Hawkman, Atom and Snapper Carr into accepting their paralyzing kisses. Once they accomplished their evil deeds for their master, these being like the lethal Lantern trio faded from existence.
During Per's previous time traveling case, he brought back to 1941 future versions of Wotan, Professor Zobar Zodiak, Sky Pirate, King Bee and the Monster (as well as Solomon Grundy, whose Earth-One counterpart was the artificial Grundy clone created by the Parasite in his war against Superman). These chronal clones existed during the same time period as their earlier version, who were unaware that their future selves were engaged in battle with the All-Star Squadron. This scheme, as with all of Degaton's schemes, failed miserably.
Years later, T.O. Morrow reappeared and disgarded the use of chornal clones, and once more manufactured Red Tornado androids modeled after the original, who turned against his master and joined the Justice Society (and later the Justice League). These Tornado androids were made of the same temporal technology as the living chrononal clones, but unlike these being they were made of silicon chips and hardware.
