The Harry Potter fandom is a large international and informal community drawn together by J. K. Rowling's Harry Potter series. The fandom works through the use of many different forms of media, including web sites, fan fiction, podcasts, fan art and songvids. Harry Potter fan fiction, stories based on the series but written by fans and distributed online, is the most searched-for subject of all fan fiction on the web, surpassing even those in the Star Trek fandom, or Trekdom. However, the fandom not only interacts online in Internet forums, but also gathers at scholarly fan conventions, tours of iconic landmarks relevant to the books and production of the films, and parties held for the midnight release of each book and film. By the fourth Harry Potter book, the legions of Harry Potter fans had grown so large that considerable security measures were taken to ensure that no book was purchased before the official release date Studies on the fandom have shown that both children and adults are fans, despite Rowling's original marketing of the books towards children aged nine to twelve.