What is Yamaguchi Opening?
The Yamaguchi rule was the opening rule of the Renju World Championships from 2009 to 2015. It works like the older RIF rule, except that the first player also sets how many fifth moves Black will offer, so a strong opening for Black can be balanced with a high number. Once the opening is over, normal renju takes over.
The first player places three stones forming one of the 26 standard openings: Black on the centre point, White inside the central 3 × 3 square and Black inside the central 5 × 5 square. They then declare how many fifth moves Black will offer. The published rule sets no upper limit; here the number is 1 to 10. The other player may then swap colours, and whoever holds White plays the fourth stone anywhere.
Black offers the declared number of fifth moves, no two of them symmetrical, and White picks one of them for Black and plays the sixth stone. With a single declared fifth move, Black simply plays it.
Rule changes
| Rule | Standard Renju | Yamaguchi Opening |
|---|---|---|
| First stones | Standard RenjuBlack plays the first stone anywhere on the board, then the players take turns. | Yamaguchi OpeningThe first player places the first three stones, Black on the centre point, White inside the central 3 × 3 square and Black inside 5 × 5, then declares 1 to 10 fifth moves. |
| Fourth move | Standard RenjuWhite plays the fourth stone anywhere. | Yamaguchi OpeningWhite plays the fourth stone anywhere, after the other player has decided whether to swap colours. |
| Colour swap | Standard RenjuNot used: each player keeps the colour they start with. | Yamaguchi OpeningOnce, after the declaration: the other player may swap colours before the fourth stone. |
| Fifth move | Standard RenjuBlack plays the fifth stone anywhere. | Yamaguchi OpeningBlack offers the declared number of fifth moves, no two symmetrical, and White picks the one that is played. A single declared fifth move is simply played. |
| After the opening | Standard RenjuBlack may not make a double three, a double four or an overline. Black wins with exactly five in a row, White with five or more. | Yamaguchi OpeningThe same, from the sixth stone on. Forbidden moves also apply to Black's stones and offered fifth moves during the opening. |
History
The Renju International Federation adopted the Yamaguchi rule for its title tournaments at its extra General Assembly in 2008, which also approved the Soosõrv and Taraguchi rules. It was the opening rule of the World Championships of 2009, 2011, 2013 and 2015, and of the Team World Championships from 2010 to 2016.
It answered the weakness of the RIF rule. With exactly two fifth moves, many of the 26 openings favour one side clearly, so top players kept returning to the same few balanced ones. Letting the opener declare more fifth moves makes the sharper openings playable again. From 2017 the World Championship moved on to Soosõrv-8.