Cells are numbered from 0, from top-left to bottom-right, and start with a white background.
Player 1's current picks have bright red top halves. After Player 2 makes a valid move, those four fills become dim red with a light gray tint.
Player 2's chosen pairs have green bottom halves. Available moves are not highlighted.
Moves and restrictions
Player 1 chooses a set R of 4 cells per round. They may select cells used by Player 2, but cannot reuse their own picks.
Player 2's cells form pairs (0, 1), (2, 3), (4, 5), …, including across row boundaries. The final unpaired cell is unavailable to Player 2.
Player 2 clicks either member of an unused pair (2i, 2i+1). At least one member must be between min(R) and max(R), inclusive; the clicked member itself may be outside that range.
A valid pick gives both cells green bottom fills and makes both unavailable to Player 2 in future rounds. Invalid picks leave the board unchanged.
A player loses when they cannot make a valid move.
Controls
− / +: decrease or increase the grid side length by two and start a fresh game. The number between them shows the current side length.
P1-auto / P2-auto: toggle computer play for the corresponding player.
Demo / Pause: start an automated game, pause it, or resume it.
Step: start a stepped demo or advance one turn at a time: Player 1's four picks, then Player 2's pair.
svg: download a vector snapshot of the current board.
asvg: download a looping animated SVG replay when a demo is paused, stepped, or finished.
gif: download a looping GIF replay when a demo is paused, stepped, or finished.
Full-view icon (left of Rules): fit the square grid within the browser window with padding. It adapts when the window is resized and preserves the game. Click again to restore the standard size.