Yonkoma is played on an even sided, square checkerboard, oriented with a dark square in the bottom left corner. The two players, Red and Blue, each have two checkers. Red’s checkers start on dark squares in two diagonally opposite corners. Blue’s checkers start on light squares in the other two corners. Figure 1 shows the starting position on the 6x6 board used in the figures. The players take turns moving their checkers, one checker per turn, starting with Red. Passing is not allowed, but if you don’t have an available move, your turn is skipped. Cycles and deadlocks cannot occur in Yonkoma. Mark Steere, in collaboration with Claude, an AI model made by Anthropic, designed Yonkoma in October 2026.
On your turn, move one of your checkers any number of squares along a diagonal, as a bishop moves in chess. A checker cannot pass through or land on another checker.
The distance between two squares is the number of king moves between them. That is, it’s the larger of the number of rows apart and the number of columns apart. Your move must do one of two things: 1) bring your moved checker closer to the two enemy checkers, so that the sum of its distances to them decreases, or 2) leave that sum unchanged and move your checker farther from your other checker.
In Figure 2, Red’s checker on b2 is at distances 2 and 4 from the blue checkers, a sum of 6, and at distance 1 from the red checker on a1. Red may move his b2 checker to c3 or d4, where the sum is 4, or to c1 or e5, where the sum is still 6 but the distance from a1 grows to 2 or 4. Red may not move it to a3, where the sum is 7, or to f6, where the sum is 8.
A crosscut is a 2x2 block of squares holding all four checkers, with the two red checkers on one diagonal of the block and the two blue checkers on the other. Whoever completes a crosscut loses.
In Figure 3, Red has just moved to e3, completing a crosscut, and losing the game. Blue has won.
I saw a scene in a movie long ago. An Asian village. Two old guys were playing a board game with a small number of checkers on it. Probably they were at the end of a game that started with many more checkers, but the image stayed with me, and I’ve always wanted a game played with a small number of checkers. Working with Claude, I was finally able to accomplish this. I suggested the goal of forming a crosscut. The move mechanism was Claude’s idea. But there was a flaw: a degenerate mirroring strategy. I fixed that by changing the goal to not form a crosscut.
Feel free to publish this rule sheet and to program the game of Yonkoma. No licensing fee or royalties are expected. However, please don’t change the name or the rules, and please attribute the game to me, Mark Steere. My other games can be found at marksteeregames.com.
Tap one of your checkers and its legal moves light up in green. Tap one of them to move. With the confirm step on, the checker is shown faintly on its new square with a dashed gold outline until you tap Confirm, and Cancel takes it back. A white ring marks the last checker moved. Undo takes back your move together with the computer’s reply, one move in two-player mode, and is greyed out when there is nothing to take back. Hint marks every winning move when the position is won, and otherwise the moves that hold out longest. Record shows the moves of the current game as text you can copy. New game asks first when a game is in progress. Computer vs computer lets you set a level for each side and watch the game play out.
Your settings and an unfinished game are kept in your browser.