Cube Chess — Frontier variant

Chess played on the surface of a 4×4×4 cube: six 4×4 panels, 96 squares in all. This implementation now enforces the rules (checks, checkmate, stalemate and legal-move generation all work), so it can be played against the computer as well as human vs human.

Panels and notation

The six panels are numbered 1 to 6. Panel 1 is the White home base, panel 6 the Black home base; panels 2–5 wrap around the sides. A square is named <panel><row><column>, with rows A–D and columns 1–4 — for example 1C2 is panel 1, row C, column 2 (the White king's starting square).

Frontier walls

Along each home base, the two edges where squares of the same colour meet are closed by a wall: no piece may move across them. These walls fence off the two base panels from the side panels along those edges, while leaving the pawns' corridors and the equator open. A rook or queen sliding along an open ring travels the full great circle (16 squares) until it is blocked or reaches a wall.

Initial setup

Each side has 16 men: 8 pawns, 2 rooks, 2 knights, 2 bishops, 1 queen and 1 king. Pawns occupy the two open edges of the home panel (rows A and D); the pieces fill the two inner rows (B and C).

Black's panel 6 mirrors this, with the king on 6B2 and its castling rook on 6B4.

How the pieces move

• Rook — slides along the cube's orthogonal great circles, turning naturally across panel edges.
• Bishop — slides diagonally across the surface, wrapping around edges. It cannot pass through a cube corner (a vertex where three panels meet): a diagonal aimed at a vertex stops there.
• Queen — rook and bishop combined.
• Knight — the usual (2,1) leap, transported across edges; it jumps over pieces but not over walls.
• King — one square in any of its (up to eight) surface directions.
• Pawn — advances straight along its meridian away from its home panel, one square at a time (two on its first move, from either home row). It captures onto the two forward diagonal squares.

Castling

Short-side castling only. White plays King 1C2→1C4 with Rook 1C4→1C3; Black plays King 6B2→6B4 with Rook 6B4→6B3. As in orthodox chess it is illegal while in, through, or into check, and the squares between king and rook must be empty.

Promotion

A pawn promotes to queen, rook, bishop or knight when it reaches row B or C of the opposing home panel (panel 6 for White, panel 1 for Black).

En passant

As in orthodox chess, a pawn that has just advanced two squares on its first move can be captured en passant by an enemy pawn attacking the square it skipped over, on the very next move only.

Special sideways move

If a pawn is carried onto one of the side panels by diagonal captures over the edges, it may step sideways off that panel onto an adjacent through-panel, so it can resume its forward march. This sideways step is allowed only in that situation.

Links

Website of the creator of the variant to play or acquire a physical game.