|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".
|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.
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.
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:
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);
Below is the explanation of some terms used in Renju.
This picture includes two unbroken rows:
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).
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.