Born as a fifth week event initiative by Dan Jurgens, the Tangent universe was based in the idea of use know superheroes names but under a totally different concept. Under that base the Atom was a nuclear-powered powerhouse, Bat-man a centuries age arthurian dark knight, Flash, a living hard-light girl, Superman, an artificially evolved human, Green Lantern, a mystical item used to bring back people from the dead among others. Other names were used for anecdotal hints: Impulse was drink, Red tornado the name of a weapon, World's Finest a webmagazine about superheroes.
There were two wave of publications and later a miniseries. They has been integrate to the new52 multiverse as earth-9
There are major historic differences between earth-9 and other earths. The Cold War is still in active; The Cuban missiles crises ended badly, with Florida and Cuba destroyed and mutating the wild life into intelligent subaquatic lifeforms; Atlanta was rebuilt and rechristened as New Atlantis; a war in Czekowzlovakia almost brought the world to WW3. Superheroes were a media sensation, higher than in regular DCU. Also the level of technology of common use was 10-15 years more advanced than the year of appearance in comics (In earth 9 tablets were popular in the late 90s).
The Tangent Universe was named earth-97 (by their year of foundation) during Infinite Crisis, when the multiverse was temporally bringed back and the Tangent universe were one of the earths among them. After the Infinite Crisis, the Green Lantern (the item) ended in new earth/earth-0 until reach Kyle Rayner, (then Ion) and later to the Justice League of New Earth. From this point, onwards is numered as earth-9.
Inside the lantern was hidden Lia Nelson, the Flash of earth-9, After founding Wally West and heping him against the Mirror Master of other earth searching help against the Superman of his earth, who had decided to take control of his world to stop his destruction because a potential nuclear war between both sides. His sucess was so great than he decided to expand his influence to the new-earth, finding opposition
With the help of the main earth justice League, a reformed earth-9 Justice League defeated Superman and put him in a prision he created for Ultra Humanite.
Isn't clear how the events of Flashpoint affected earth-9 and which is the exact status of their reality and if their encounter with the people of earth-0 still was remembered.