Chess

How it is played

1. Standard FIDE chess: castling, en passant, promotion, check and checkmate. White is a seat drawn from the match's seed; the preset names the variant and the clock.
2. Draws are automatic, nobody claims them: stalemate, the third occurrence of a position (same pieces, side to move, castling rights and a legal en-passant capture; in Crazyhouse also the same pieces in hand), 100 half-moves without a capture or a pawn move (a checkmate on that move still wins; not in Crazyhouse), and material with which neither side could ever win.
3. Chess960: the back ranks are one of 960 start positions, drawn from the match's seed (number 518 is the standard start). Castling puts the king and rook where they go in standard chess (g and f, or c and d); every square between them and their landing squares must be empty, and the king may not be in check or cross an attacked square.
4. King of the Hill: a king that reaches d4, e4, d5 or e5 wins at once. Material is never a dead draw: a king can always walk.
5. Three-check: the third check given wins. Only two bare kings are a dead draw.
6. Atomic (lichess's rules): every capture is an explosion that removes the captured piece, the capturing piece and every piece but a pawn on the eight squares around. Kings cannot capture. Exploding the enemy king wins at once, and a move that would explode your own king is illegal. Kings that touch cannot check each other.
7. Crazyhouse (lichess's rules): a captured piece goes to the captor's hand (a promoted piece as a pawn), and a move may instead drop a piece from your hand on any empty square: pawns not on the first or last rank. There is no 50-move rule; a position seen three times within 256 half-moves is a draw.
8. Horde (lichess's rules): White has 36 pawns and no king, Black the usual army. White's pawns on the first rank may also step two squares (and cannot be taken en passant then). Black wins by capturing every white piece, White by checkmate. A side with pieces but no legal move is stalemated: a draw.
9. Los Alamos 6×6: no bishops, no castling, pawns step one square only (so no en passant), and promote to a queen, rook or knight. Gardner 5×5: no castling, pawns step one square only.
10. Capablanca 10×8: each side also has an archbishop (moves as a bishop or a knight) and a chancellor (a rook or a knight). The king castles three squares, to the i or c file, the rook landing beside it. Pawns may promote to an archbishop or a chancellor too.
11. Grand chess 10×10 (Christian Freeling's rules): the rooks start in the corners, the other pieces on the second rank, the pawns on the third; no castling; pawns step two squares from the third rank. A pawn may promote on the eighth or ninth rank and must on the tenth, but only to a piece its side has lost (one queen, chancellor and archbishop, two rooks, bishops and knights at most); a pawn with nothing to become cannot move to the tenth rank.
12. Time: each move has the preset's time per move, and a bank (per preset) pays for any move that takes longer. When both run out the game is lost on time, or drawn if the opponent could never win. A seat to move may resign. The winner takes both stakes; a draw returns each its own.