Action Comics was the second longest running comic book series of all time, with only Detective Comics (Volume 1) having been published longer. As of June 2011, it had surpassed the 900 issue mark. Aside from its venerable publication history, Action Comics is more famously known as the birthplace of Superman, the most well-known superhero in the world. Superman made his first appearance in June of 1938 in the title sequence story from Action Comics #1.
For the first several years of publication, Action comics existed as an anthology series, featuring multiple twelve-page stories featuring a variety of characters ranging from Congo Bill to Zatara. By the 1950s, the popularity of Superman had reached such a zenith, that the title became informally regarded as a "Superman" title. By the 1970s, Action Comics featured Superman exclusively with occasional eight-page back-up stories spotlighting various characters such as the Atom or Aquaman. For a year in 1969 and 1970 the back-up story was home to the Legion of Super-Heroes.
In the mid-1980s, Action Comics endured several format changes. In September of 1986, the title went on a four-month publishing hiatus, in order to prepare for the Post-Crisis Superman relaunch event. The editors of DC comics wanted to produce three regular ongoing Superman titles. Beginning in January of 1987, Action Comics became a Superman team-up book, following the format of the late comic title, DC Comics Presents. It maintained this status until the celebrated 50th anniversary landmark 600th issue. From issue #601–642, Action Comics became a 48-page anthology series again, however this time issues were distributed on a weekly basis and the title was officially changed to Action Comics Weekly. With issue #643, Action Comics Weekly reverted to its original monthly publication, and the title was changed back to Action Comics. With the return of a monthly publishing schedule, Superman stories began to follow a chapter-based continuity, with storylines moving from one title to the next. In order to provide an easily accessible reading order for consumers, the DC editorial staff began using a secondary numbering system displayed on the cover of each issue. These continuity markers were informally known as the "triangle" numbers, and were recycled at the end of each publishing year. Every Superman comic book published from the mid to late 1990s (including Annuals and specials) possessed a sequential chapter number on the cover. Eventually, writers began containing their storylines to single titles again and the DC editors ceased providing the chapter markers on the covers. This practice was resurrected for the two year New Krypton storyline