Nijigen Ura (二(に)次元(じげん)裏(うら), lit. “2D/Back”), also known as Nijiura (虹(にじ)裏(うら)),¹⁾ is a group of Futaba Channel boards for topics in the two-dimensional space, with the 'ura' (裏) kanji signaling that 'adult-oriented' and 'off-topic' behavior is permitted to some degree, given that the rules allow it.

In 2002, the first 'image bulletin board' (画像掲示板) was set up on March 12, then a 'guro image' (グロ画像) board was made after enough complaints on March 14,²⁾ but then the original board became '3D' (三次元) while a new '2D' (二次元) board was set up on March 16.³⁾ By June 2002, a number of 'ura' (裏) boards were made, initially with the reputation of being a 'buffer zone' for newcomers for that specific board.

Over time, the board's culture grew, the 'first memes' surfaced, and Toshiaki makes his debut. Then in 2005, the board was split across several servers: 'img' and 'dat' held the traditional format, while the rest carried modern features (e.g. default names, image replies, etc.). In 2010, 'nov' was closed. Several years later, 'jun' and 'dec' moved from /b/ on December 8, 2018 and July 30, 2020 respectively.

• Traditional Nijiura (hidden)
  • Nijiura Img
  • Nijiura Dat
• Modern Nijiura
  • Nijiura May - The fast-paced, popular board, since they used to be first in the list.
  • Nijiura Jun - Mostly focused on erotica, sometimes has original content.
  • Nijiura Dec - For casual discussions and live anime commentary (“viewing parties”).

“Toshiaki” (としあき) is the default name for most Nijiura-related boards. Back in April 2003, there was a troll named “Toshiaki” who kept pestering users to share a specific doujinshi.⁴⁾ Instead, the users mocked him by posting with his name, until said troll was permabanned, but the administrator was presumably amused by the situation and decided to set “Toshiaki” as the default name, creating a sense of unity in Nijiura.

He is usually represented with the monochromatic 'self-portrait' of a gloomy, bespectacled man with a beard, initially uploaded on December 12, 2003,⁵⁾ and many users ended up relating to his self-portrait for years. Then on March 7, 2020, the original artist who drew said self-portrait stepped up, allegedly turning out to be a mangaka named 'Takashi Tonagi' (十凪高志, Tonagi Takashi).⁶⁾

• The default subject is 'Munen' (無念, lit. “No Thoughts”) and the default name is 'Toshiaki' (としあき).
  • For the traditional boards, they don't have names, so they resort to some form of '「」'.
• Nijiura was never a 'rule-free' board. This seems to have been a misconception or mistranslation over the aforementioned 'ura' (裏) concept, which 4chan and its later clones managed to perpetuate.
• There are a number of Nijiura refuges, but I'm not sure it's a good idea to list them, and I don't want people to get blindsided if one refuge follows the ancient 'read-only member' superstition or believes all non-Japanese IPs are proxies attempting to waste their bandwidth.

• Futaba Channel § See also - Moved here to reduce redundancies.

• Nijiura on Pixiv Encyclopedia
• Nijiura on Niconico Encyclopedia
• Futaba Channel § Nijigen Ura on Japanese Wikipedia