Yonkoma 10x10

Yonkoma is played on an even sided, square checkerboard, oriented with a dark square in the bottom left corner. The two players, White and Black, each have two checkers. White's checkers start on dark squares in two diagonally opposite corners. Black's checkers start on light squares in the other two corners. The players take turns moving their checkers, one checker per turn, starting with White. Mark Steere, in collaboration with Claude, an AI model made by Anthropic, designed Yonkoma in October 2026.

OBJECT OF THE GAME

A crosscut is a 2x2 block of squares holding all four checkers, with the two white checkers on one diagonal of the block and the two black checkers on the other. Whoever completes a crosscut loses.

PLAY

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.

LEGAL MOVES

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. The following positions show possible moves from a selected checker.

Passing is not allowed, but if you don't have an available move, your turn is skipped, as shown below. White moves and black has no legal moves so passes, white then has one final move to complete the cross cut, and black wins!

Cycles and deadlocks cannot occur in Yonkoma.

DESIGN NOTES

"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." Mark Steere.