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The house of Honinbo was one of the Four houses in the Edo Period in Japan. Today the Honinbo tournament is the oldest of the current Japanese professional titles and one of the three most important big titles.
During the Edo period the Honinbo (本因坊, Hon'inbō) was the head of the Honinbo school (originally founded by Honinbo Sansa). Of the four traditional go schools, the Honinbo was the most prestigious and successful one. The last hereditary Honinbo, Shusai, gave (or sold) his title to the Nihon Ki-in so as to turn it into a tournament title. This is a list of the historic Honinbos, including the heirs who did not succeed to the Head of the House. (Heirs will not have a number before their use name.)
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Note: The 17th and 19th Honinbo are the same person, and the 16th and 20th are the same, too. Also from 1864-1884 Shuei had been head of the Hayashi school.
The Honinbo title is the oldest Go tournament in the world and in some ways still the most prestigious in Japan, though the Kisei and Meijin big titles have larger prize funds. The winner's prize is ¥32,000,000 currently, sponsored by the Mainichi Shinbun. The games can be found on their website (Japanese).
Due to the importance of the Honinbo as one of the "big three" titles in Japan (along with the Kisei and Meijin), there are several paths of automatic rank advancement through it in the Nihon Ki-in new promotion system. Qualifying for the Honinbo league warrants a promotion to 7-dan, winning the league to challenge for the title promotes to 8-dan, and finally winning the title itself gives an immediate 9-dan promotion.
As with each of the three Japanese big titles, the previous year's title holder is challenged by the winner of a league. In the event of a tie during the league, there is a playoff between the top two players. Entry to the league is gained through a series of preliminary tournaments. The title is decided in a best of seven match, where each player is given eight hours of thinking time over a two day period.
The 1st Honinbo Tournament started in June 1939 with 27 players participating in a set of four successive knockout mini-tournaments in which first place yielded the player 6 points, second place 5 points, down to 1 point for players eliminated in the first round. Of the 27 players, eight were thus selected to play in a league to determine the top two players. The eight players were: Sekiyama Riichi, Kato Shin, Go Seigen, Maeda Nobuaki, Kubomatsu Katsukiyo, Suzuki Tamejiro, Segoe Kensaku, and Kitani Minoru. All these games used komi. The top two then played a six game series of no-komi go. Black won all games, the result being tied at 3-3. The title was awarded to Sekiyama Riichi based upon his better performance in the qualifying rounds.
The title Honorary Honinbo is given to those players who have previously won the Honinbo title five years in a row. The Nihon Ki-in has been numbering those players who have qualified for the Honorary Honinbo title as a continuation of the list of hereditary honinbos, as follows:
 By custom the Honinbo would adopt his successor.
 The 1st Honinbo tournament in 1941 involved a bitter dispute over the introduction of komi. The preliminary events used a komi of 4.5 but once the participants in the title match were decided, Kato Shin succeeded in requiring that the title match be played without komi and that the Mainichi Shinbun report his view that "Komi go is not go".
Amusingly enough after the vocal opposition to komi, Black won all six games of the title match. It had been previously specified that in the event of a tied match, the player with the better previous results would be declared the winner. Karma caught up to Kato Shin with a vengeance, and based on those previous results Sekiyama Riichi became the first non-hereditary Honinbo title holder.
 Sekiyama Riichi became ill during game two of the 2nd Honinbo tournament in 1943. He fought through the illness, but collapsed unconscious by the board on day two of the second game. He was then forced to forfeit both the game and the match.
 The second game of the 3rd Honinbo tournament in 1945 is the famous atomic bomb game, which took place in Hiroshima and was interrupted on its third day by the atomic bomb being dropped there. Fortunately the game had been moved to the outskirts of the city at police insistence so there were only minor injuries caused by flying glass. Perhaps most famously, the game was then set up again and concluded after lunch the same day.
The first six games resulted in a 3-3 tie, and a seventh game playoff was intended to be played with 4.5 points komi. Due to the end of the war though, there were no resources to set up this game. A year later in 1946 they met again (finally) for a three game playoff match, which Iwamoto Kaoru won 2-0.
 In the aftermath of the war, the title match was reduced to five games for the 4th Honinbo tournament in 1947.
 A book was actually written about the 26th Honinbo, titled The 1971 Honinbo Tournament appropriately enough. It details how Ishida Yoshio (at age 22) entered the Honinbo League for his first time, won it, and then went on to defeat the established Honinbo Rin Kaiho in the title match. All of Ishida's games from the league and title match have extensive commentary in the book.
 In the 60th Honinbo tournament game one, there are some interesting moves. See at 60th Honinbo Title Match 1st Game Move 43. There's also some discussion about the position at the end of the first day of game four.
 Recovered from 0-3, winning the title.