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Metric Chess is chess played on a 10x10 board, with two new pieces per side and a few new rules to go with them. It runs in the browser at metric-chess.500foods.com, where you can play a friend or the computer.
What is different from standard chess:
- Board: 10 files by 10 ranks, 100 squares, with files and ranks numbered rather than lettered.
- Pieces: 20 per side. The back row runs trebuchet, rook, knight, bishop, queen, king, bishop, knight, rook, trebuchet, with 10 pawns in front.
- Trebuchets: a new piece that jumps exactly three squares in any direction, over anything in the way. It cannot attack anything closer.
- Pawns: may move one or two squares forward on any move, not just the first. They still capture one square diagonally forward.
- Promotion: a pawn reaching the far rank can also become a trebuchet or a king.
- Heir kings: a promoted king is an heir. Only your last remaining king can be checked or checkmated; heirs can be captured like any other piece. No king may move next to another king, of either colour.
- Castling: as well as castling with a rook, the king can castle with a trebuchet on either side.
- 10-move rule: after 10 full turns with no capture, the side with fewer pieces wins. Equal counts are a draw.
Everything else works as in standard chess: checkmate wins, and stalemate, threefold repetition and kings-only positions are draws. The computer opponent is built on Fairy-Stockfish, and the game can be installed and played offline. The full rules and source are on GitHub.