How to play Fusor

The idea

Fusor is chess, with one extra operation. Every unit on the board is a part — pawn, knight, bishop, rook, queen, or king — and friendly parts can occupy the same square. A fused piece moves as the union of its parts. A rook fused with a bishop moves like a queen. A knight fused with a pawn jumps and also steps like a pawn.

You still win by checkmate. The king never fuses with anything, so the royal piece stays a single king for the whole game.

The starting position

Games start from the normal chess position. White at the bottom, pawns on the second rank, pieces on the first. Every piece begins as a single part. Nothing is fused until you choose to fuse it.

Time controls are 3, 5, or 10 minutes for each player, with no increment. Flag fall loses, the same as on a chess clock.

Moving

On your turn, pick one of your pieces and move it using any non-empty subset of its parts. A single piece only has one subset: itself. A fused piece can move as the whole stack, or send only some of its parts.

Click a fused piece again to cycle which parts will move. Your opponent does not see that choice until you play. Dragging always moves the whole piece.

The destination has to be a square at least one of the moving parts could reach. Sliding parts (rook, bishop, queen) stop at the first occupied square. Knights jump. Pawns move forward onto empty squares and capture diagonally.

Fusing

Land the parts you are moving on a friendly piece to fuse. The parts join that piece. The combined piece now has every move of every part in it.

A piece can hold at most three parts. The king cannot fuse, and nothing can fuse onto the king. Fusion onto your own piece is not a capture, so it does not reset the fifty-move clock unless a pawn moved.

Disconnecting

Send only some of a fused piece's parts to a new square. The parts you didn't choose stay behind. That is a disconnect: one piece becomes two, and you can redeploy a bishop that had been glued to a rook.

The parts that stay behind do not move, so they keep the square. The parts that leave follow their own movement rules from the square the stack was on.

Captures, en passant, and promotion

Moving onto an enemy piece captures it, the same as in chess. Only the parts you sent make the capture; parts left behind stay on the original square.

En passant works for any piece that contains a pawn. If an enemy pawn (or a fused piece containing a pawn) double-steps past you, you may capture it diagonally onto the skipped square. The moving subset has to include the pawn.

A pawn that reaches the last rank may promote to a queen, rook, bishop, or knight, or stay a pawn. That includes a pawn that is riding inside a fused piece. You pick the promotion when you play the move.

The king, check, and castling

You may not leave your own king in check. A fused piece gives check with every part it contains: a rook-knight checks along ranks and files and also with a knight jump.

Castling is the normal chess move, and only with an unfused king sitting on its home square. The rook can be fused, as long as it still has a rook part on its home square and the squares between are empty. You cannot castle out of, through, or into check.

How a game ends

Checkmate wins. Stalemate is a draw. So is threefold repetition of the same position, the fifty-move rule, agreement, and king versus king with nothing else on the board.

Resignation and running out of time lose. If a king's square is captured out from under it, that is a win by king capture — the same side that would have delivered checkmate.

Human games are rated. Both players' Elo moves, and the result lands on the leaderboard. Games against the bot are practice: they are saved on your profile under a separate practice rating and do not touch your ranked Elo.

Playing a person

The live queue pairs you with the closest-rated player on the same clock, and falls back to a bot if nobody is waiting. You do not have to be online at the same moment. Challenge a friend, send the link, and the game starts when they open it.

After a game, rematch swaps colors and uses the same clock. A guest can play immediately. Saving an email keeps that same rating and the same game history.