The Soosõrv opening rule is a renju opening rule. It was proposed by Estonian player Ants Soosõrv.
The sequence of moves implied by the rule follows.
This rule gives an average variety of new playable variants in a good number of openings, especially white-oriented, but openings that are very strong for black (like 2D, 2I, 4I, 7I, 4D etc.) don't become playable.To solve this problem the Soosõrv-N advancement was proposed and certified by RIF.
Soosõrv-N opening rule is an advancement of Soosõrv opening rule. When the white player puts the 4th move and declares the number of fifth moves, the number has to be not less than 1 and not greater than N, instead of the default value 4 for N in the original Soosõrv opening rule.[2]
Depending on the value of N this rule gives an average to big variety of new playable variants in a growing number of openings. Soosõrv-5 is very close to Taraguchi concerning a number of playable positions. Soosõrv-8 makes all 26 renju openings available.
Soosõrv opening rule was an official opening rule for European Renju Championship in 2008 and a couple of minor international tournaments.
Soosõrv-N opening rule was certified by Renju International Federation in 2011 after a proposal from Russian Renju Association. In 2015, Soosõrv-8 opening rule was decided to be the opening rule for the Renju World Championship since 2017.[3]