ChessMerge rules

ChessMerge uses ordinary chess movement, check, and checkmate, with the changes below. These are the rulings implemented by the game.

Merging and splitting

Move onto a friendly, unmerged piece to combine the two on one square. The destination must be legal for the moving piece.
A square can hold at most two friendly pieces. A merged pair cannot merge with a third piece.
On a later turn, move either component under its own movement rules. The other component stays behind, so the pair splits.
The component that splits away may land on another friendly, unmerged piece and merge there in the same move.
The pair may move together using either component's movement pattern. Both pieces arrive on the destination square.
Kings cannot merge. Capturing a merged square captures both pieces on that square.

Pawns

Pawns move one square forward. They do not get a two-square first move.
Pawns capture one square diagonally, as in ordinary chess.
A pawn may merge by moving one square forward onto a friendly, unmerged piece.
There is no en passant.

Promotion

A pawn that reaches the last rank promotes to a queen, rook, bishop, or knight.
A pawn may split from a merged pair onto the last rank and promote. Its partner remains on the original square.
If a pair containing a pawn moves together onto the last rank, promotion replaces both pieces with the selected piece.
Moving only the non-pawn component onto the last rank does not trigger promotion.

Castling

The king and the original corner rook must still have castling rights. The path must be clear, and the king cannot castle out of, through, or into check.

The rook stays home

Another piece may merge into the corner rook and later split away. The rook itself never moved, so castling rights remain.

The rook is still merged

Castling is allowed. The king moves normally, and the whole rook pair moves to the rook's castling square.

The rook component moves

Moving the rook alone or moving the whole pair from its corner permanently removes castling rights on that side.

The corner rook is captured

Castling rights are lost even when the rook was one component of a captured pair.

Check and game end

You cannot make any move that leaves your king in check. This includes splitting a pair and moving a pair together.
The game records checkmate and stalemate. Challenges are untimed.
The app does not currently adjudicate threefold repetition, the fifty-move rule, or insufficient material.