documentation of de facto standards for chess variants
This is an overview of standards used for chess variants. Most standards are taken over from chess with minor extensions and modifications, but the differences often are not documented, so this page tries to fill this gap.
The Portable Game Notation is a standard for recording chess games, see its specification. For chess variants it can be naturally extended by generalizing the move and position notation, i.e., SAN and FEN. The main addition is the Variant header tag containing the capitalized variant name in order to specify the variant the game record is using.
The Forsyth-Edwards Notation for standard chess is specified in the PGN standard. For chess variants FEN can be naturally extended by allowing arbitrary piece characters and board sizes. For specific rule modifications some extensions and modifications are required, such as for check counting, S-Chess gating, counting rules, and the like.
See the FEN subpage for details.
The specification of the Shogi Forsyth-Edwards Notation is part of the USI protocol specification.
The Extended Position Description for chess variants does not have any variant-specific modifications other than applying all the changes as for FEN. Apart from that the EPD specification defined in the PGN standard for chess applies. It is recommended to add a variant operation in order to specify the variant name.
The Short/Standard Algebraic Notation for standard chess is specified in the PGN standard. For chess variants there are several extensions in order to represent special moves, such as piece drops and gating.
@ before the target square, e.g., P@f7.+, e.g., +P@a3./, e.g., Qd2/E. If gating happens to a square other than the origin square, the square is indicated after the gating piece, like Qa4/Pd4. This is also the case when the gating would otherwise be ambiguous, such as in case of castling due to the option of gating on king or rook, e.g., 0-0/Eh1.The pure coordinate notation is specified as part of the UCI protocol (also here). For chess variants it requires extensions in order to describe piece drops, gating, and shogi-style piece promotions.
@, e.g., P@f7.+, e.g., +P@a3.+, e.g., b2h8+. Corresponding demotions are denoted with a -.d1d2e. For castling if gating happens on the rook square, it is encoded as rook captures king, e.g., h1e1h. In games where the gating happens on arbitrary squares, e.g., game of the amazons, the square is indicated after the gating piece, like a4a5pd5.The Universal Chess Interface is a chess engine protocol specified here (older version). For chess variants it only requires an additional convention which UCI option to use to set the variant, which is UCI_Variant, and it uses the generalized FEN and coordinate notation as described above. The UCI_Chess960 option for chess variants is used as a general switch to Chess960 style castling rules and notation in both the FEN (KQkq -> AHah) and coordinate notation (e1g1 -> e1h1). Note that as for standard chess UCI a position command is required before starting a search, i.e., updating UCI_Variant is not required to set up the new variant yet.
The Universal Shogi Protocol is a dialect of the UCI protocol for Shogi defined in the USI protocol specification.
The Universal Chinese Chess Interface is a dialect of the UCI protocol for Xiangqi. It skips the name and value keywords in the reporting of options and instead requires option names not to contain spaces in order to disambiguate parsing of tokens. It uses a move coordinate notation using ranks 0-9 instead of 1-10.
The UCI-cyclone protocol is a modification of the UCI protocol for Xiangqi. It uses a move coordinate notation using ranks 0-9 instead of 1-10 and it skips the position keyword in commands to set up the board position.
There are several specifications of the Chess Engine Communication Protocol here, here, and here. It by the specification already supports chess variants.