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# Folding Magnetic Chess

I did not invent this variant, and I cannot find it in Pritchard's encyclopedia or anywhere else.

Many chessboards do fold in half. And if it's magnetic, then the pieces can stick to both halves, so you can play the variant as is. Folding magnetic sets are hardly new or rare -- I've had a cheap one for 30+ years. I don't remember this variant being called anything particular, we just "played on a folded board".

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## Rules

Fold a standard board along the line between the d-file and e-file, to get a double-sided 8x4 board. A piece on top can move past the edge of the board, in which case it flips and continues underneath. And vice versa.

## Notes

I marked it as a 3-D game because it's 8x4x2, but I also marked it as a 2-D game, because there is an equivalent game that you could play on a standard 8x8 board. You might find the 2-D version more convenient, even if you do have a folding magnetic chess set -- the 3-D version just shows you what the rules should be.

• The left and right edges are considered to be joined, so that a-file and h-file are adjacent (just like in cylindrical chess).
• But also, the left half of the top edge joins to the right half of the top edge, in such a way that continuing up from a8 goes to h8, b8 goes to g8, c8 goes to f8, and d8 goes to e8, and vice versa. The bottom edge joins to itself as well, so that going down from a1 goes to h1 and so on. That is, the a-file and h-file form a single 16-step loop, as do b-g, c-f, and d-e (just like the 4x16 rings in circular chess).

You can check that the 2-D variant is equivalent to the 3-D variant.

You can check that from 56 squares the king has 8 distinct moves. From 8 corner squares (a1,d1,e1,h1,a8,d8,e8,h8), two of the eight moves end up on the same square, and one of them ends up where it started. For example, from e8 its moves are, starting from 3 o'clock and going clockwise: f8, f7, e7, d7, d8, "d9", "e9", and "f9". The first five are the usual moves, "f9" is c8, "e9" is d8 (redundant), and "d9" is e8 (null move).

This also gives a way to confirm that this variant is not equivalent to various warped-board variants -- for example, there are spherical and torus variants on this website with 64 squares, but the king has 8 distinct moves from all 64 squares. (The spherical variants on this website are not "simply connected". There are spherical variants in Pritchard's encyclopedia which are simply connected compact surfaces without boundary, but none of them have 64 squares.)

I also posted a variant id question on chess stack exchange, but I got silence.

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Author: Deep Thought.

Last revised by Fergus Duniho.

Web page created: 2021-08-07. Web page last updated: 2021-08-28﻿