Crossed Corners

Rules

Crossed Corners is a four-in-a-row game with a head start. The 4×4 board begins with four pieces already in place: owns the upper-left and lower-right corners. owns two cells in the middle instead — the center pair of its own diagonal. Twelve squares remain open.

Take turns claiming an open square. Get four of your pieces in a row —across, down, or diagonally— to win.

Both players already have two pieces on their own diagonal, so each player starts one step away from a threatening four-in-a-row. Note that the two-piece headstart is arranged differently for each player.

If all the squares are filled without a four-in-a-row, the game is a draw. Who goes first alternates with each new game.

New here? Medium is a good place to start. It plays a strong but not flawless game, giving you room to discover the strategy before taking on Hard.

Controls

Columns are lettered a through d, left to right. Rows are numbered 1 through 4, counted from the bottom up (matching Four-in-a-Row's convention). The two starting corner cells and the two starting center cells are already filled and can't be played into.

With a mouse or touchscreen: click any open square to place your mark there.

With a keyboard: type a coordinate into the box below the board — "b3," for example, is column b, row 3 — then press Enter or Place.

Type status (or just the start of it, like "sta") and press Enter or Place at any time — even when it isn't your turn — to have the whole board read back to you.

Roots

Crossed Corners is a 4×4 cousin of Tic-Tac-Toe, seeded with a couple of pre-placed pieces so the board isn't a guaranteed draw the way plain Tic-Tac-Toe is. Scale the board up further and add more in-a-row requirements and you land on Gomoku, the ancient n-in-a-row game this whole family ultimately descends from.

Its sibling, Four-Patch, keeps this lineage but adds a second win shape on top.

Under the hood

A small game with an interesting problem

Crossed Corners starts with four pieces already on the board. Each player has two "anchor" pieces, leaving only 12 open squares. That creates an interesting strategic race: each player begins with half of a diagonal already completed, while the remaining rows, columns, and diagonals compete for the squares in between. You might expect that the anchors should be symmetrical, but that would leave very little real competition on the board, with testing confirming draws were produced in roughly 73% of medium-vs-medium games. While perfect play produces a draw regardless of who moves first, Self-play on the asymmetrical board with the medium computer opponent produced a much healthier draw rate of about 53%.

How the computer plays

Crossed Corners is small enough that the computer can examine the game tree rather than relying entirely on a heuristic guess.

Hard plays from the exact game-theoretic solution, so it doesn't merely estimate which move looks good— it can determine the eventual result of the position with perfect play.

Medium uses the same underlying game logic but deliberately gives the player more opportunities by limiting how far ahead it searches and by introducing occasional play imperfections.

Aggressive is intentionally different. It doesn't try to solve the whole position. It first looks for an immediate threat it must block; otherwise, it concentrates on advancing its own strongest line.