Characters from DC Comics were originally suggestive of each existing in their own world, as superheroes never encountered each other. This was soon changed with alliances being formed between certain protagonists. Several publications, including All-Star Comics (publishing tales of the Justice Society of America), Leading Comics (publishing tales of the Seven Soldiers of Victory) and other comic books introduced a "shared-universe" among several characters during the 1940s until the present day.
Alternative reality Earths had been used in DC stories before, but were usually not referred to after that particular story. Also most of these alternative Earths were usually so vastly different that no one would confuse that Earth and its history with the so-called real Earth. That would change when the existence of another reliable Earth was established in a story titled "Flash of Two Worlds" in which Barry Allen, the modern Flash later referred to as Earth-One (the setting of the Silver Age stories) first travels to another Earth, accidentally vibrating at just the right speed to appear on Earth-Two, where he meets Jay Garrick, his Earth-Two counterpart.
Johns, Geoff; Morrison, Grant; Rucka, Greg; Waid, Mark (w), Giffen, Keith; Barrows, Eddy; Batista, Chris; Justiniano; McKone, Mike; Olliffe, Patrick; Robertson, Darick (p), Geraci, Drew; Lanning, Andy; Ramos, Rodney; Robertson, Darick; Wong, Walden (i). "A Year in the Life" 52, no. 52 (July 2007). /wiki/Geoff_Johns
McAvennie, Michael; Dolan, Hannah, ed. (2010). "1960s". DC Comics Year By Year A Visual Chronicle. London, United Kingdom: Dorling Kindersley. p. 103. ISBN 978-0-7566-6742-9. This classic Silver Age story resurrected the Golden Age Flash and provided a foundation for the Multiverse from which he and the Silver Age Flash would hail. {{cite book}}: |first2= has generic name (help)CS1 maint: multiple names: authors list (link) 978-0-7566-6742-9
Fox, Gardner (w), Infantino, Carmine (p), Giella, Joe (i). "Flash of Two Worlds!" The Flash, no. 123 (September 1961). /wiki/Gardner_Fox
Siegel, Jerry (w), Shuster, Joe (p), Shuster, Joe (i). "The Origin of Superboy" More Fun Comics, no. 101 (January–February 1945). /wiki/Jerry_Siegel
Hamilton, Edmond (w), Swan, Curt (p), Fischetti, John; Kaye, Stan (i). "The Mightiest Team in the World!" Superman, no. 76 (May–June 1952). /wiki/Edmond_Hamilton
Irvine, Alex "1950s" in Dolan, p. 73: "Jimmy Olsen got his own adventures in Superman's Pal Jimmy Olsen #1. A comic remarkable for its inventiveness and longevity, it ran for 163 issues." /wiki/Alexander_C._Irvine
Irvine "1950s" in Dolan, p. 77: "The Martian called J'onn J'onzz debuted as a regular feature in Detective Comics #225. 'The Strange Experiment of Dr. Erdel', by writer Joe Samachson and artist Joe Certa, gave the origin for the lonely Martian Manhunter."
Irvine "1950s" in Dolan, p. 80: "The arrival of the second incarnation of the Flash in [Showcase] issue #4 is considered to be the official start of the Silver Age of comics."
Herron, France (w), Papp, George (p), Papp, George (i). "The Rainbow Archer" Adventure Comics, no. 246 (March 1958). /wiki/France_Herron
Irvine "1950s" in Dolan, p. 90: "Wonder Woman's origin story and character was given a Silver Age revamp, courtesy of writer Robert Kanigher and artist Ross Andru."
Fradon, Ramona (p)Fradon, Ramona (i)"How Aquaman Got His Powers" Adventure Comics, no. 260 (May 1959). /wiki/Ramona_Fradon
Irvine "1950s" in Dolan, p. 95: "DC had decided to revamp a number of characters to inject new life into the genre. Writer John Broome and artist Gil Kane ensured that Green Lantern got his turn in October [1959]'s Showcase #22."
Fox, Gardner (w), Kubert, Joe (p), Kubert, Joe (i). "Creature of a Thousand Shapes!" The Brave and the Bold, no. 34 (February–March 1961). /wiki/Script_(comics)
McAvennie, Michael; Dolan, Hannah, ed. (2010). "1960s". DC Comics Year By Year A Visual Chronicle. London, United Kingdom: Dorling Kindersley. p. 103. ISBN 978-0-7566-6742-9. This classic Silver Age story resurrected the Golden Age Flash and provided a foundation for the Multiverse from which he and the Silver Age Flash would hail. {{cite book}}: |first2= has generic name (help)CS1 maint: multiple names: authors list (link) 978-0-7566-6742-9
McAvennie "1960s" in Dolan, p. 103: "The Atom was the next Golden Age hero to receive a Silver Age makeover from writer Gardner Fox and artist Gil Kane."
McAvennie "1960s" in Dolan, p. 109: "The two-part 'Crisis on Earth-One!' and 'Crisis on Earth-Two!' saga represented the first use of the term 'Crisis' in crossovers, as well as the designations 'Earth-1' and 'Earth-2'. In it, editor Julius Schwartz, [writer Gardner] Fox, and artist Mike Sekowsky devised a menace worthy of the World's Greatest Heroes."
McAvennie "1970s" in Dolan, p. 146 "It was taboo to depict drugs in comics, even in ways that openly condemned their use. However, writer Denny O'Neil and artist Neal Adams collaborated on an unforgettable two-part arc that brought the issue directly into Green Arrow's home, and demonstrated the power comics had to affect change and perception.
Wein, Len (w), Dillin, Dick (p), Giella, Joe (i). "The Unknown Soldier of Victory!" Justice League of America, no. 100 (August 1972). /wiki/Len_Wein
Wein, Len (w), Dillin, Dick (p), Giella, Joe (i). "The Hand That Shook the World" Justice League of America, no. 101 (September 1972). /wiki/Script_(comics)
Wein, Len (w), Dillin, Dick (p), Giella, Joe; Giordano, Dick (i). "..And One of Us Must Die!" Justice League of America, no. 102 (October 1972). /wiki/Script_(comics)
McAvennie "1970s" in Dolan, p. 153: "Following his debut in House of Secrets #92 in 1971, the Swamp Thing grew into his own series, albeit with a reimagining of his origins by writer Len Wein and artist Bernie Wrightson."
Thomas, Roy; Thomas, Dann (w), McFarlane, Todd (p), Montano, Steve (i). "Last Crisis on Earth-Two" Infinity, Inc., no. 19 (October 1985). /wiki/Roy_Thomas
Conway, Gerry (w), Staton, Joe (p), Machlan, Mike (i). "The Final Crisis" Justice League of America, no. 244 (November 1985). /wiki/Gerry_Conway
Wolfman, Marv (w), Pérez, George (p), Ordway, Jerry (i). "Death at the Dawn of Time" Crisis on Infinite Earths, no. 10 (January 1986). /wiki/Marv_Wolfman
Gerber, Steve (w), Veitch, Rick (p), Smith, Bob (i). "Phantom Zone: The Final Chapter" DC Comics Presents, no. 97 (September 1986). /wiki/Steve_Gerber
Manning, Matthew K. "1980s" in Dolan, p. 220: "In 'Whatever Happened to the Man of Tomorrow?', a two-part story written by Alan Moore and illustrated by Curt Swan, the adventures of the Silver Age Superman came to a dramatic close."
Moore, Alan (w), Swan, Curt (p), Pérez, George (i). "Whatever Happened to the Man of Tomorrow?" Superman, no. 423 (September 1986). /wiki/Alan_Moore
Moore, Alan (w), Swan, Curt (p), Schaffenberger, Kurt (i). "Whatever Happened to the Man of Tomorrow?" Action Comics, no. 583 (September 1986). /wiki/Script_(comics)
Johns, Geoff; Morrison, Grant; Rucka, Greg; Waid, Mark (w), Giffen, Keith; Barrows, Eddy; Batista, Chris; Justiniano; McKone, Mike; Olliffe, Patrick; Robertson, Darick (p), Geraci, Drew; Lanning, Andy; Ramos, Rodney; Robertson, Darick; Wong, Walden (i). "A Year in the Life" 52, no. 52 (July 2007). /wiki/Geoff_Johns
Straczynski, J. Michael; Davis, Shane (October 2010). Superman: Earth One. DC Comics. p. 144. ISBN 978-1401224684. 978-1401224684
Straczynski, J. Michael; Davis, Shane (November 2012). Superman: Earth One Vol. 2. DC Comics. p. 136. ISBN 978-1401231965. 978-1401231965
Johns, Geoff; Frank, Gary (July 2012). Batman: Earth One. DC Comics. pp. 144. ISBN 978-1401232085. 978-1401232085
"DCU in 2010: Welcome to Earth One". DC Comics. December 7, 2009. Archived from the original on September 3, 2012. Retrieved September 1, 2012. https://web.archive.org/web/20120903165935/http://www.dccomics.com/blog/2009/12/07/dcu-in-2010-welcome-to-earth-one
Jackson, Leah (August 1, 2011). "New Batman Arkham City Bonus Costumes Revealed – Earth One, The Animated Series, Batman Beyond, And More". G4. Archived from the original on January 13, 2013. Retrieved September 8, 2012. https://web.archive.org/web/20130113111103/http://www.g4tv.com/thefeed/blog/post/715006/new-batman-arkham-city-bonus-costumes-revealed-earth-one-the-animated-series-batman-beyond-and-more/