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Ren is an abstract strategic game for two players, played on a 10x10 board. “ren” means “group”.
Players take turns placing one of their stones at a time in any vacant square in an effort to create groups and score points. The player with most points at the end of the game wins.

On their turn, a player may:
a)pass (if both players pass consecutively, the game ends), or
b)place a stone of their color on the board. (either on an empty space, or replacing an opponet’s weak stone - see below)

scoring.
Players attempt to create groups of their color. When they do, they are immedeately scored.
Scoring groups are:
four in a row - 1 point
six in a row - 2 points
four in a box - 1 point
six in a box (2x3 or 3x2) - 2 points
nine in a box (3x3) - immedeately wins the game. If the player that achieved the nine in a box was already winning, add 10 points to their score. If the player was losing, then swap scores and add 10 points to the scoring player. The game immedeately ends.

four-in-a-row, horizontal and vertical. (1pt. each)

a horizontal six-in-a-row (black - 2pts.), and a four-in-a-box (white - 1pt.).

six-in-a-box, horizontal for black and vertical for white (2pts. each)

nine-in-a-box for white, wins the game. (+10pts. for the winner)

Note that a single placement can score more than one group. For example, a player may score a four-in-a-row and a six-in-a-box at once and add 1+2=3 points to their total.
Also note, if a five-in-a-row, a seven-in-a-row or any other non-scoring, single-row group is formed, it is not worth any points. An eight-in-a-box (2x4) -or other non-scoring box groups- will however be scored as the sum of the included largest scoring groups created by the scoring stone.

multiple scoring: by playing at a, black scores 3 points. 2 for the horizontal six-in-a-box and one for the vertical four-in-a-row.

by playing at b, white scores 3 points. A horizontal 4-row, vertical 4-row and a four-in-a-box, for 1 point each.

playing at a, black scores no points. five-in-a-row is not a scoring group. Black does not score 1 point for the four-in-a-row included in the five.

by playing at c, white scores 2 points, for the horizontal six-in-a-row.

b gives white 3 points, 1 for the vertical four and 2 for the vertical six-box. Eight-box is not a scoring group.

a scores 1 for black (a-1-2-3 is a four-box but a-2-3-4-5-6 does not score, as it is neither a four-, nor a six-row.

b gives white 5 points, 1 for the vertical four and 4 for two vertical six-boxes. (1-2-3-b-4-5 and 3-b-4-5-6-7). Eight-box is not a scoring group.

weak stones (rui).
When a player scores one or more groups the stones adjacent to the scoring stone (orthogonally or diagonally) that are of the same color are said to be “rui” - meaning “weak”. On the next move, the opposing player may either play on any empty space on the board as always, or may replace one of the enemy’s rui stones with one of their own. There is one exception to this rule - a “niban” stone cannot be rui, therefore it cannot be removed (see below).

rui limit:
Only up to three weak stones can be replaced by both players in succession. Make sure both players count how many rui stones have been replaced in succession, so that both know when the rui limit has been reached.

By playing at 1, black scores, so any black stones surrounding 1 orthogonally or diagonally (in the positions marked x) become rui (weak). The stone at 1 is not rui. After the move, white may either play on any vacant space as before, or may remove a black rui stone and play a white in its place.

capturing.
A single stone that is orthogonally surrounded by enemy stones or the edges of the board on 3 of its four sides is said to be in “atari”. This means that it is in danger. A stone in atari can be removed on the next move of the attacking player, if he/she totally surrounds the stone.

If a single stone is orthogonally surrounded on all four sides by enemy stones or the edges of the board, as a result of a surrounding player’s placement, will be removed from the board and returned to its owner (it is captured). Only a single stone can be captured, not groups of stones.
Important rule: it is not permitted to make a capture by replacing a rui stone. The rui stone may be replaced, but no capture is made.

stone 1 is in “atari”. It has white stones on its left, right and top, leaving only the bottom side open.
stone 2 is surrounded by black stones on the left and right and the edge of the board at the bottom, the only open side being the top.
stone 3 has a white stone on top and the edge of the board on its right and bottom, leaving only the top side free.

white captures the black stone marked “1” on the previous diagram by placing a white stone at the bottom side of 1 and totally surrounding the black stone. The black stone is returned to its owner.

black decides to play at the center of the white “cross”, which is a perfectly legal move (and a good idea). It does not matter that the new black stone is surrounded.

If the stone that achieved the capture can itself be captured on the next move (is in “atari” itself, as a result of its placement), then the previously removed stone is not only removed, but also replaced with one of the capturing player’s stones (this stone is called “niban” - meaning “second”). This way a player not only removes an enemy stone, but places two of their own stones on the board. This way of placing two stones at once is called “nibai” - meaning “double”.

black is about to capture the white stone marked 1. When black captures, black’s newly placed stone will be in atari itself, as 1 will be returned to white. The “nibai” rule comes into play.

black captures, and places a second stone in the position of the captured stone, according to the nibai rule. This is a devastating move, as black is certain to score next move, no matter how white answers.

If a stone that preforms a nibai also creates a scoring group, the niban stone (which is not the scoring stone) is not considered rui (weak), therefore it cannot be removed by the other player on the following move.

as in the previous example, black is about to capture the white stone marked 1. As before, the “nibai” rule comes into play.

black captures, and places stones 1 and 2, scoring a four-in-a-row. Since black scored, the stones adjacent to the scoring stone (1), become weak (rui). However, the second stone placed (niban, marked 2) does not become rui.

game end:
the game ends when:
a)both players pass consecutively
b)a nine in a box is achieved by either player - the nine-in-a-box scorer always wins.
c)there is no more space on the board

variations:
a)increase the rui limit to five.
b)a player with a six point lead immedeately wins the game.

It is recommended that a match comprises of two games. The player that starts first on the first game, should go second on the next. Scores from the first and second game should be added to determine the final winner of the match.

An 11x11 go or pente board and stones is an excellent way to enjoy ren. A 9x9 go board will result in an 8x8 ren game, which is a bit smaller than recommended, although still perfectly playable. Go players will notice the borrowed go terminology and capturing technique. Remember, however, that only a single stone can be captured in ren and it is a perfectly legal move to place a stone in a position with no liberties. Ren is played on the squares, not the intersections. Players that are not familiar with go are highly encouraged to look into it!