3139: Chess Variant

2025-09-30T13:44:00Z

2A01:119F:2A5:8D00:ACCB:1DF3:3A75:E8DD:

{{comic
| number = 3139
| date = September 8, 2025
| title = Chess Variant
| image = chess_variant_2x.png
| imagesize = 310x344px
| noexpand = true
| titletext = The draw-by-repetition rule does a good job of keeping players from sliding a tile back and forth repeatedly, but the tiles definitely introduce some weird en passant and castling edge cases.
}}

==Explanation==
A {{w|Sliding puzzle|sliding puzzle}} is a puzzle with movable pieces that challenges players to slide the pieces around the board to get them into a certain pattern or to move a certain piece into a certain position. Patterns can be anything from a completed image to a series of numbers. One of the most common variants, the {{w|15 puzzle}}, is a square board with 15 square pieces (usually numbered 1 through 15, to be placed in obvious order, but can also feature segments of a larger picture that needs to be correctly assembled) and one empty space in a 4×4 grid. The goal is to order the numbers (or reassemble the picture) without lifting any piece, only sliding adjacent pieces into the empty space. [[Randall]] contemplates making a {{w|Chess variant|variant of chess}} in which 2×2 sections of the board can be moved around, possibly as an alternative to moving your own pieces. It is possible that “sliding number chess puzzle” is a pun on actual {{w|chess puzzles}} in which pieces are set up in a position and the player must find the best move or sequence of moves in that position.

A similar concept exists in {{w|Three-dimensional chess#Star Trek Tri-Dimensional Chess|Star Trek 3D chess}}. Although there's no official rule set by the show creators, the rules were invented by Star Trek fans. In this variant, the board has several 2×2 "attack boards" that can be moved around. For a more prosaic analog, the game {{w|Labyrinth (board game)|Labyrinth}} uses a board composed of tiles that players use to rearrange the playing arena, and features a similar prohibition against reversing the change made by the previous play.

The title text states that because of the {{w|threefold repetition}} rule in chess, sliding a tile back and forth will result in a draw, just as would already happen with the moving of the ''pieces'' back into an overall state of the board. This may discourage unimaginative 'stalling' play by one player, in allowing the other player to claim a draw and avoid a loss. However, this rule would probably lead to more draws, as it allows the player in the losing position to move tiles in an attack that could easily be avoided by moving the tile back. Thus, it gives a small extra advantage to the player in the losing position, as the player who is winning is not likely to draw their position. However, it's also mentioned that there are logistical challenges involving ''en passant'' pawn capture and castling with the tiles involved.

''{{w|En passant}}'' is a unique interaction between adjacent pawns of opposing sides. Normally, pawns can only move straight ahead, one space at a time, and can only capture pieces that are placed one square diagonally forward. However, an unmoved pawn can choose to advance two spaces rather than the usual one. If such a pawn thus bypasses the threatened area of an opposing pawn, that opposing pawn may then capture the pawn that moved two spaces ''en passant'', treating it as vulnerable (and retroactively capturable) at the mid-move moment that it had only travelled one space forward. Since the sliding chess puzzle is separated into 2×2 boards, a pawn positioned on one of those pieces would be able to move two spaces rather than the usual one from its transported location. It could be argued that a pawn moving directly past an opposing pawn on a moving 2×2 board segment could be vulnerable to ''en passant'' capture. Possibly, the capturing pawn would end up where the captured pawn was originally destined, it having attacked the 'stationary' but shifting pawn, mid way through the movement of the board tile, and then effectively completed the act of being carried onward. Also possibly, this style of ''en passant'' would apply to pawns effectively carried two tiles backwards/sideways past a suitably placed opposing pawn, transitions that a pawn could not otherwise make by its usual mode of movement.

{{w|Castling}} is a move involving the king and rooks. Normally, the king can only advance one square at a time in any direction, while a rook can move on either axis but cannot pass through other piece. If neither the king or rook have been moved in the current game, there are no pieces between them, and no opposing piece threatens the king's whole movement, a player may choose to "castle". This (in a normal setup) notionally involves moving the king two spaces towards a chosen rook (at the extreme left and right edges of the back rank upon which the king starts nearly in the middle of) and then placing that rook directly in the space the king moved over. It could be argued that a king and/or rook placed on respective 2×2 boards of the sliding puzzle, at least one of which has been shifted since the start of the game, have not themselves moved and thus should be eligible to castle from their resulting relative positions, with suitably modified repositioning rules.

It is also left unclear whether the wider-ranging pieces are allowed to effectively move through the missing/virtual spaces in the board where there currently is no tile, beyond merely being unable to ''end'' their move in the current 'hole' where no traditional chess squares exist at that moment. It also brings up the question of whether pawns promote if the tile that they are on gets moved to the last rank of tiles with the pawn on the last rank of squares. If so, who would get to choose which piece to promote, if the player whose pawn it isn't moved the piece?

One interpretation of the game shown is that white played e4, black e5, continuing with Nf3 and Nc6, then white played d4 (all normal moves, so far, the [https://lichess.org/opening/Scotch_Game Scotch Game]). In response, black slid the puzzle-square to the right to make white’s knight on the rim 'dim', and decentralize white’s pawn.

An early example of Randall depicting a 'movable' fragment of chessboard was used in [[839: Explorers]]. Though that one was of size 3×3, and had become entirely separated from the 'home board' (perhaps not even being originally part of it, having initially been assembled adjacent to it) and under its own motive power.

==Transcript==
:[A chessboard is shown with the white pieces at the bottom of the screen. The pieces are illustrated in the basic design of standard computer chess. The chessboard is divided into 16 2×2 sliding squares with the e3-f4 sliding square currently being moved to the g3-h4 spot. Otherwise the opening is a standard Scotch Opening, with the pieces in the e3-f4 tile like how they are supposed to be in a scotch opening.]
:[Caption below the panel:]
:Sliding number puzzle chess

{{comic discussion}}<noinclude>

[[Category:Chess]]

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3139: Chess Variant