A sogo was an official challenge match during the Edo Era in Japanese go.
When there was a serious dispute such as who should become Meijin it could be settled by a challenge match between the disputants or their representatives. One of the most famous such matches was that between Gennan Inseki and Honinbo Shuwa when Gennan attempted to get himself appointed Meijin Godokoro after Jowa's resignation. The Honinbo house objected and proposed a 20-game match between Gennan and Shuwa. Shuwa won the first game so convincingly that Gennan withdrew his application.
1645-1653 - Honinbo Sanetsu vs. Yasui Sanchi: a six-game sogo for the title of Meijin. Played on tagai-sen, Black won all games, resulting in a 3-3 draw. As a result, neither player was promoted to Meijin.
1668-1676 - Honinbo Doetsu vs. Yasui Sanchi: Sanchi had managed to become Meijin Godokoro by means of political manoeuvering. Doetsu objected, asking for a Sogo. It was granted on the condition that if Doetsu lost he would be exiled forever.
It was a sixty-game affair, with the handicap changing after a lead of six wins or four in succession. The match started on jou-sen, the handicap for a two-rank difference, being Doetsu 7 dan and Sanchi formally the Meijin, i.e. 9 dan.
After 16 games the score was 9-3-4 jigo in Doetsu's favour. The handicap was lowered to sen-ai-sen, but Doetsu won 3 games out of the next four, losing only as white. Sanchi had enough of it; he abandoned the match and resigned as both Meijin and Godokoro.
1737-1740 - Honinbo Shuhaku vs. Inoue Shunseki Inseki: Shuhakus application for promotion to 7 dan was blocked by Hayashi Incho Monnyu (whose application to Meijin had been opposed by the Honinbo and Yasui Houses some time before). Shuhaku challenged Incho to a sogo but the latter refused on grounds of his illness. Shunseki then stepped in to play in his place.
After two and half years only 8 games had been played: Shuhaku was ahead 4-3-1 jigo, but then he suffered a serious stomach haemorrhage. The match had to be suspended. Shuhaku died shortly afterwards, in early 1741.
1766-1767 - Honinbo Satsugen vs. Inoue Shunseki Inseki: a twenty-games sogo to decide the Meijin. After the traditional prearranged jigo in the first game, Satsugen won the next 5 games and applied for a change in the handicap. A series of mutual objections were made by the players regarding the conditions for changing the handicap, until the match was suspended.
1840 - Shuwa vs. Inoue Gennan Inseki: Gennan's application for Meijin Godokoro was opposed by Honinbo Josaku, who put forward his heir Shuwa for a sogo. Only one game was played though, as Shuwa's 4-point win with black was so convincing that Gennan abandoned the match and rescinded his application.
1846-1847 - Honinbo Shuwa vs. Shusaku: a seventeen-game series between teacher and pupil. Shusaku won 13-4, but Shusaku played at jou-sen throughout because he refused to play white out of respect for his teacher; as a result, it is not known which of the two players is stronger.
1851 - Shusaku vs. Sekiyama Sendaiu: The strongest amateur of the Edo period, samurai Sekiyama Sendaiyu, arranged for Shusaku to come to Matsushiro for a twenty game series of teaching games. The games were played in only twenty days, one each day, with Sekiyama using 5-3 in each of them and Shusaku never playing an opening twice.
1853 - Shusaku vs. Ota Yuzo: a friendly thirty-game match (sanjubango). It started on even, and it took Shusaku 17 games to force a change in the handicap. The final score was 13-7-3 jigo in Shusaku's favour.