Yasui Chitoku (安井知得, Nakano Isogoro, Nakano Chitoku, Yasui VIII Senchi, 1776-1838 in Izu, Japan) was a Japanese, professional 8-dan. He was the eighth head of the Yasui house, lifelong rival and friend of Honinbo Genjo, and considered one of the Four Sages. Yasui Chitoku was of Meijin strength, as was Honinbo Genjo, but out of respect for one another they both stayed Jun-Meijin, as there could only be one Meijin. After 1803, he never took Black again, except when playing against Genjo.
Yasui Chitoku was born Nakano Isogoro in 1776 in Izu, Japan. He was the son of a fisherman, but moved to Edo at a young age. He entered the Yasui house and quickly rose to 2-dan. During this time he had already met and played his future rival and best friend Miyashige Rakuzan (later to become Honinbo Genjo).
In 1800, Isogoro became heir to the Yasui house and changed his name to Nakano Chitoku. During the same year he was also promoted to 5-dan and played his first castle game against Genjo. In 1801, he became 6-dan; 1804, 7-dan; and by 1806 he had already become of Meijin strength. In 1814, Yasui Senchi Senkaku retired and Chitoku became head of the Yasui house and was promoted to 8-dan. As was tradition of the house, Chitoku became Yasui Senchi the VIII (or Yasui VIII Senchi Chitoku). However, out of ease modern usage simply refers to him as Yasui Chitoku.
Chitoku would have likely retired around the time of Genjo, 1827, were it not for the sudden machinations that Honinbo Jowa and Inoue Genan Inseki initiated in their attempts to become Meijin. The situation left Chitoku in the middle of a murky power-struggle that he may have wanted to keep his heir and son, Shuntetsu (aka Yasui Sanchi), out of. As such, he remained active until his passing in 1838.
Yasui Chitoku is said to have had the most 'restrained' style in go history. The Japanese term for this is 'shibui,' and it refers to a particular aesthetic of simple, subtle, and unobtrusive beauty. His games appear to be simple overall, but include subtle details that balance simplicity with complexity. He played with a thick and solid style that aimed for no vulgarity or showiness and prevented trickery from the opponent. He was a master of shinogi.
At least 87-records remain between Chitoku and Genjo. Of those games with known results Chitoku won five more than Genjo, but also took Black seven more times. Keeping in mind that there was no komi for White during those times, it seems reasonable to conclude that the two were perfectly matched. One of the key highlights for their games was that both players played with contrasting styles. Their first game was played on 1788-05-11, when they were still known by their birth names (Nakano Isogoro and Miyashige Rakuzan), and Genjo won the that game. Their second game was played five months later on 1788-10-31, and Chitoku won that game. This back-and-forth became typical for their games. Records show of a jubango played between the two around 1790 with a result of 5-wins, 4-losses and 1-jigo to Chitoku. However, most of the game records are lost.