Platonic Chess

Platonic Chess is played on a 10x10 board. It consists of ten chess pieces based on the five Platonic solids, and a row of ten Pawns on the third rank.

Setup

Pieces

Pawns

There are ten pawns at the third rank, and they move as in Orthodox Chess.

Platonic Solids

There are two duplicate pieces for each Platonic solid type. The moves for each Platonic solid piece are as below:

Tetrahedron

A Tetrahedron has 4 surfaces, so it can only reach 4 squares.

It moves one square orthogonally like the Wazir.

It is a royal piece like the King in orthodox chess, and can be put in check.

Cube

A Cube has 6 surfaces, so it can only reach 6 squares.

It moves one square diagonally like the Ferz and slides up to two squares forward.

Octahedron

An Octahedron has 8 surfaces, so it can only reach 8 squares.

It moves diagonally like a Bishop , but only up to two squares.

Dodecahedron

A Dodecahedron has 12 surfaces, so it can only reach 12 squares.

It has the combined moves of Knight and Dabbabah.

Icosahedron

An Icosahedron has 20 surfaces, so it can only reach 20 squares.

It moves like a Rook, but only up to four squares or like an Alfil.

Rules

Promotion

A Pawn can capture as a Platonic solid piece in addition to its original moves upon reaching the enemy side:

• 6th rank -moves and captures as a regular Pawn, can also capture as a Tetrahedron
• 7th rank - moves and captures as a regular Pawn, can also capture as a Cube
• 8th rank - moves and captures as a regular Pawn, can also capture as an Octahedron
• 9th rank - moves and captures as a regular Pawn, can also capture as a Dodecahedron
• 10th rank - moves and captures as a regular Pawn, can also capture as an Icosahedron

The ranks given above are from the point of view of the promoting player.

The Pawn's additional capture ability is linked to the enemy rank it is on, and is not cumulative. A promoted Pawn can only move passively as a regular Pawn, and capture one square diagonally forward. It can also capture as one of the Platonic solids depending on which enemy rank it is on (see above). When a promoted Pawn moves away from the enemy side by capturing as a Tetrahedron backward, it reverts to a regular Pawn.

When a Pawn reaches the last rank, it can no longer move or capture forward. Its only legal move is to capture up to four squares orthogonally sideways or backward by sliding, or exactly two squares diagonally backward by leaping. Therefore, unlike in orthodox chess, pushing a Pawn to the last rank may not be the best strategy, and it could be advantageous to keep the Pawn in another rank on the enemy side, depending on the positions of the other pieces on the board.

Castling

A Tetrahedron can perform the castling move with an Icosahedron on the same side. The final position is the same as the positions of the equivalent pieces (King and Rook) after Queenside castling in orthodox chess.

Checkmate

The game is won when either Tetrahedron is checkmated.

Fork Checkmate

When the Tetrahedrons are forked by an attacking piece that cannot be captured in the next move, the game is won since the two Tetrahedrons cannot be moved away from check at the same time.

• A Dodecahedron can easily perform a fork checkmate if the Tetrahedrons are right next to each other orthogonally or diagonally. Other fork checkmate positions by the Dodecahedron are also possible when the Tetrahedrons are two or three squares apart in various configurations.
• A Cube can perform a fork checkmate if the Tetrahedrons are right next to each other or one square apart orthogonally or diagonally.
• An Octahedron can perform a fork checkmate when the Tetrahedrons are one or three squares apart orthogonally or diagonally.
• An Icosahedron can fork the Tetrahedrons if it lies between the two in the same rank or file (provided the Tetrahedrons are not more than seven squares apart), or when it is one square apart diagonally from one Tetrahedron and one square apart orthogonally or diagonally from the other Tetrahedron.

A fork checkmate can only be prevented by capturing the attacking piece.

Skewer Checkmate

When the Tetrahedrons are skewered by an attacking piece that cannot be captured in the next move and there is no way to block the skewer attack, the game is also won since both Tetrahedrons cannot be moved away from check simulateously.

• If the Tetrahedrons are skewered by an Octahedron along the diagonal and the Octahedron cannot be captured in the next move, the game is won.
• An Icosahedron can perform a skewer checkmate if the Tetrahedrons are next to each other or separated by one square orthogonally. If the Icosahedron is one or two squares away, a skewer checkmate can be blocked by placing an intervening piece in between the Icosahedron and the Tetrahedrons.
• A Cube can perform a skewer checkmate if the Tetrahedrons are right next to each other vertically. The Cube must be guarded by another piece and is threatened only by the Tetrahedron in front of it. (Any other piece that threatens the Cube can prevent the skewer checkmate by capturing it.)

A skewer checkmate can be blocked by placing an intervening piece between a skewering Icosahedron and the Tetrahedrons. It can also be prevented by capturing the attacking piece.

Notes

How to play using commercially available equipment

You will need:

• 1 x International Checkers set with a 10x10 board
• 4 x Platonic dice set
• 2 x Chess set (Optional)

The checker pieces can be stacked up to show that they are promoted when they are on the enemy side (see Rules section above):

• 6th rank - stack of 2 checker pieces
• 7th rank - stack of 3 checker pieces
• 8th rank - stack of 4 checker pieces
• 9th rank - stack of 5 checker pieces
• 10th rank - stack of 6 checker pieces

Since checkers pieces can be used as pawns on the Platonic chess board, the chess sets are optional. If regular Pawn pieces are used, you can colour the board using highlighters to demarcate the five ranks on each side, keeping the Pawn pieces as they are when promoted. Their additional capture ability would be indicated by the different colours associated with each rank.

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Platonic Chess

Last revised by Albert Lee.