Rules
Crumble is an easy-to-make turn-based strategy game for 2 players (also playable online). One player plays the black pieces, the other plays the white. Black plays first. The pieces always entirely fill the game board, so that a ‘capture’ means that pieces become the opposing color, not that they are taken out of play. On each turn, a player has various options to affect the size, shape, and color of the pieces on the board. The goal for each player is to connect their pieces into chains that touch all four sides of the board. A player may announce "crumblemate" if their opponent has no defense against loss on the next turn. It is considered a courtesy in casual games to say "crumble" when you threaten to win on your next turn.
A won position for White:
A 'chain' of pieces, either for winning or capturing, consists of same-colored pieces that touch on more than just a corner. A ‘chain’ for swapping colors must further share a complete side of both pieces.
Squares and half-squares are legal pieces, but not any other type of rectangle or other shape.
As part of making a legal move, pieces may be cut smaller and smaller, or joined larger and larger, as long as they retain a legal shape. There are no constraints as to the size of the pieces.
The starting position is a 6x6 checkerboard pattern, with Black occupying the upper left corner.
The above diagram is not the empty board, it's the starting position itself, with each white and black square being one piece.
Taking A Turn
Each player's turn follows the same pattern:
(Required) Perform either a split or a join, as defined below.
(Optional) Perform a swap as defined below.
Winning the game or capturing pieces are things that happen automatically as the above actions are performed.
Splits
A split divides some number of pieces in half.
You can only split your own pieces.
A split may apply to an arbitrary number of pieces.
A square may only split into two half-squares, and a half-square may only split into two squares.
A split must always trace a single straight line, either vertical or horizontal.
While a split may not continue through an opposing piece, it may continue along the edges of neighboring pieces (see the examples below).
Examples:
Note that the final example above is illegal because the split-line passes through an opposing piece.
Note that although you can split multiple pieces on a single turn, you can only perform a single split on your turn: you can't split some pieces over here, and some pieces over there.
Joins
Joining merges multiple pieces together into one.
You can only join your own pieces.
Any group of pieces of the same color, that have an outline that is the shape of a legal piece, may join into that piece.
You may join an arbitrary number of pieces, but only into one single piece.
Note that the final two examples are illegal. In the first case they do not form a perfect square or half-square, and in the second they attempt to join pieces together into more than one resulting piece.
Swaps
Swaps involve one piece exchanging color (not position!) with an opposing piece. It is the only way to change the configuration of color on the board. Without swapping, neither side can win.
A swap is optional and may only be done immediately after a split or a join, during the same turn
A swap must begin with either the newly joined piece, or with one of the newly split pieces.
A swapping piece and its destination piece must share a complete side.
In the examples above, the first is legal because the white and black pieces share a complete side. In the second, neither piece shares a complete side with the other. In the third, the black piece shares a complete side with the white, but the white piece does not.
Long Swaps
On the player's same turn, it is legal to continue to swap your piece through any number of the opponent's pieces, so long as the above rules are observed. This is called a 'long swap'.
Here are the steps of the above long swap, all of which were done as part of a single turn:
Notice that the white piece did not leave a trail of white pieces in its wake. It merely passed through a 'swappable region' of black pieces.
Also notice that a 'chain' (used for capturing and winning) and a 'swappable region' are not necessarily the same. While all swappable regions are chains, not every chain is a swappable region.
When performing a swap, each pair of opposing pieces swaps colors in turn. During a game you may skip the intermediary steps of a swap, but they are still understood to have occurred. Instead of performing each intermediate step of a swap, it is considered a courtesy to tap each piece involved in a long swap with your fingertip, to inform your opponent of the path taken. This is relevant when determining if someone has won the game or performed any captures. See the next sections for details.
Winning The Game
If at any point during a swap, a chain of pieces belonging to either player meets the winning condition of touching all four sides of the board at the same time, then that player immediately wins. This also means that you can lose the game in the middle of your own turn.
Capturing
If at any point during a swap, a loop of chained pieces belonging to either player forms around some of the opponent's pieces, those pieces immediately become the same color as the capturing chain of pieces. Capturing is not optional - if the conditions of a capture are met, the capture always occurs.
A capture does not mark the end of a swap. After capturing, the player may continue to swap through additional opposing pieces.
Either player's pieces may be captured on any given turn.
If the swapping piece is captured, it may not continue to swap, and that is the end of the player's turn.
A capture cannot occur against the edge of the board:
It is legal to backtrack a swap. This is useful for performing multiple captures:
You may even backtrack through the starting position of the swapping piece.
Drawn Games
Both players may agree at any time to declare a game drawn. But there are situations where a game is automatically drawn.
Three Draw Turns
This draw rule is similar in spirit to chess's threefold repetition rule, but the translation to Crumble requires a knowledge of 'renewables', and specifically of the 'draw turn'. The draw turn is described towards the bottom of this description of renewables.
The rule is expressed like this: If there are three 'draw turns' in a row, even if the target piece is different in each 'draw turn', the game is declared a draw.
One Big Piece
This rule is most likely to come into play in smaller order crumble games such as a 2x4 starting position, but the rule applies to all versions of the game.
The rule is expressed like this: If one player has a single piece that touches two opposite sides of the board, then that player may claim a draw at any time.
The rationale behind this rule is that, by creating a single piece that completely blocks all opposing paths that might connect the other two sides of the board, it is now impossible for the opponent to win, so long as that piece exists. Thus the player with that piece may choose to continue to play for a win using their remaining pieces, or they may declare the draw.
It is theoretically true that the player with the big piece, if all their other piece were captured by the opponent, might be forced to split their big piece and thus reopen the possibility that their opponent might win. But for the defender to create the big piece is enough of an achievement to justify the draw if they choose. And since there is no other circumstance where any player might be forced by the rules to split a particular piece in crumble, there is some justice in not forcing them to do so now.
Takebacks
Before play begins, players may agree on any rule they like regarding taking moves back. In tournament play, no takebacks are allowed.
However, in casual play, a common agreement is to take back any blunder that allows the opponent to win immediately on their next turn. Beginners are strongly encouraged to adopt that agreement when playing more experienced players, as it allows the game to proceed all the way to crumblemate.