Renju

Overview
Renju is a two-player abstract board game that belongs to N-in-a-row family of games. Renju is a professional variant of traditional Gomoku that includes different restrictions for making the game fair. It was named Renju by Japanese journalist Ruikou Kuroiwa in December 6, 1899 in a Japanese newspaper "Yorozu chouhou".

Objective
The objective of the Renju is to place five stones in an unbroken row (horizontally, vertically or diagonally). The first one, who achieves this goal, wins the game.

Board

Renju is played on a square board with orthogonal lines (15x15 intersections). The middle square (5x5 intersections) is highlighted because it has a special meaning in the game.

Play

A game begins with an empty board.

Each player has an allocated color, usually Black and White.

Black plays first, putting the first Black stone at the middle of the board. Then the White player makes the second move by placing the first White stone on one of the eight intersections adjacent to the first Black stone. Then the Black player makes the third move by placing the second Black stone inside the middle square (5x5). After this both players place one stone per turn on any unoccupied intersection of the board.

For making the game fair (i.e. reducing the advantage of the first move) some restrictions apply to possible moves of the Black player:

  • "Overline" - the Black player is not allowed to place 6 or more Black stones in a row.
  • "Double four" ("4x4 fork") - the Black player is not allowed to place a stone that builds two separate "Fours".
  • "Double three" ("3x3 fork") - the Black player is not allowed to place a stone that builds two separate "Threes".

The White player has no any restrictions. In addition the White player wins a game by forming an "Overline"  (6 or more White stones in a row);

Terms

Below is the explanation of some terms used in Renju.

  • "Unbroken Row" - a group of stones of the same color, which are located on the same horizontal, vertical or diagonal line with no opponent stones or empty intersections between them:

This picture includes two unbroken rows:
one unbroken row of 3 Black stones
and one ubroken row of 4 White stones.

  • "Five" - an unbroken row of 5 stones.
  • "Overline" - an unbroken row of 6 or more stones.
  • "Open Four" - a row of 4 stones that can turn into "Five" in two different ways by adding a single stone of the same color:

The Black "Four" highlighted with the red color is "Open Four" - placing a Black stone on any of the two transparent red squares will turn it into "Five"

The Black "Four" highlighted with the green color is NOT "Open Four" - it is forbidden to the Black player to form "Overline" and therefore the Black player cannot place a Black stone on the top transparent green square. So there is only one way to turn this "Four" into "Five" (placing a Black stone on the bottom transparent green square) and therefore it is NOT "Open Four".

However, the White player is allowed to form "Overline", so the White "Four" highlighted with the blue color is "Open Four" - there are two ways to turn it into "Five" (or "Overline", which is the same for the White player).

  • "Four" - a row of 4 stones that can turn into "Five" by adding a single stone of the same color in a single way. An example of "Four" was given on the previous illustration (the Black "Four" highlighted with the green color)
  • "Three" - a row of 3 stones that can turn into "Open Four" by adding a single stone of the same color:

The White "Three" highlighted with the blue color is a real "Three" - placing a White stone on the transparent blue square will turn it into "Open Four".

The Black "Three" highlighted with the green color is a real "Three" - placing a Black stone on any of the two transparent green squares will turn it into "Open Four".

However, the Black "Four" highlighted with the red color is NOT "Three" - it cannot be expanded to form an "Open Four" because any of the expansions will lead to forming an "Overline", which is forbidden for Blacks. 

Variants
  • "Classical Renju with an opening sequence"
    This is the common variant played on professional tournaments. It differs from Classical Renju by having a special opening sequence:

    1. The 1st player places two Black stones and one White stone on the board according to the rules of Classical Renju.
    2. The 2nd player now chooses whether to play Black or White.
    3. White player places the 2nd White stone on the board.
    4. Black player places two Black stones on the board.
    5. White player removes one of the two Black stones from the previous move.
    6. White player places the 3rd White stone.

    After this the game continues as in the Classical Renju.
     
  • "Free Renju"
    This is a simple variant of Renju that is played in some parts of the world. It doesn't include any restrictions used in Classical Renju. The first move (1st Black stone) is made at the middle of the board. The 2nd move (the 1st White stone) is made inside the middle square (5x5). The 3rd move (the 2nd Black stone) is made outside the middle square. After that the game continues without any restrictions. A player must place exactly 5 stones in a row in order to win a game. A player does not win by placing 6 or more stones in a row.

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