Shifted Square Chess, also known as SSQ, needs more than one page to explore its possibilities.
Here we present Partial Information SSQ as well as the closely related games of Pickle and Nick Danger Chess.
In Partial Information Single Shifted Square Chess (also known as SPITQ, following the lead of newspaper in the era of John Nance Garner in avoiding censorship), the added and subtracted locations are unknown to the players.
This is a very small amount of unknown information, and in fact if one plays SPITQ by itself the game is probably not very interesting; however, it is perhaps remarkable for being the partial-info game with the least unknown info; that is, it is probably the most least partial.
In Pickle Chess, no square is removed; instead, the square chosen from the normal board area is linked to the new location! Thus, any piece on the linked square exists in two places at once because the square exists in two places at once.
This is not exactly shifting a square, but it it somehow closely related, and it is a logical consequence of Shifted Square Chess, and so it is treated here.
Suppose that e4 is removed, and x5 is added; now the board looks like this:
8 . * . * . * . * 7 * . * . * . * . 6 . * . * . * . * 5 @ * . * . * . * . 4 . * . * @ * . * 3 * . * . * . * . 2 . * . * . * . * 1 * . * . * . * . x a b c d e f g h zThe square e4, marked with an @, is the square at e4 and it is also the square at x5. Thus, after 1. e2-e4, a7-a6 attacks the Pawn.
Other than that, there are no special rules or restrictions. A piece on a linked square can make any legal move available from either location of the square, and of course when it does so it moves off the linked square and goes back to the humdrum experience of existing in only one place.
Simple Pickle Single Square Chess is not a terribly interesting game, but it is at least a simple demonstration of the idea of squares that exist in two locations.
Therefore the idea comes to mind of playing a game on two boards, and the problem is how to link the squares. This question took me more than a week to answer, and so you must understand that I could write a thousand lines about the different possible rules I considered for this game.
Rule 1: Two chessboards are used. At the start of the game, the first chessboard is completely normal and the second chessboard is completely empty and unreachable.
Rule 1a: There is no reason for the two chessboards to be the same shape and size, except that the simplest form of the game must be when both boards are 8x8. Because the different games produced by using different board sizes and shapes should have different names, rule 1a specifies that the game called simply "Nick Danger Chess" must use two 8x8 chessboards.
For example, suppose that board 0 is normal, but board 1 is just 2x2. Isn't that an interesting game?
Rule 1b: Two boards. If you use three boards, or an infinite number of boards, you are playing a different game that needs its own name.
Rule 1c: The King is restricted to the first chessboard.
This rule is unfortunate, but it is necessary because otherwise the King would be easy to check but almost impossible to checkmate.
A side-effect of this rule is that the King cannot be checked from any other board, and therefore may Castle across or onto a square that has a link to an attacked square on another board.
The "flat King" rule might also be good in 3D Chess.
Rule 1d: Moving the King to a linked square breaks the link.
Without this rule, there would need to be another rule stating whether or not you may move to the other location when the King stands on a linked square; for example K on 0e1 which is linked to e1 on board 1, can you move to 1e1?
Rule 2: The rules of Avalanche Chess are used, but instead of advancing an enemy Pawn, one must link a square in the second part of one's move.
Note: According to Avalanche rules, you cannot leave your K in check in the first part of your move and use the second part to get out of check; I cannot think of a variant where linking a square would block check, but breaking a link could stop check...
Rule 2a: Balanced. In other words, it's probably a good idea to forbid White from linking any squares on move 1.
Rule 2b (deprecated): After all possible links have been made, the players take turns removing links; but see 2c for an alternate, and better, rule.
Rule 2c: Twinkle Danger Chess: After all possible links have been made, the players have the choice of adding or removing a link, but of course may not undo the opponent's most recent make/break.
The choice of adding or removing is called "Twinkle", as per the article in Nost/Algia which introduced Avalanche Chess along with six other closely-related games; in Twinkle Chess, the second part of one's move either adds or removes an enemy Pawn.
Rule 2c1: When an occupied square is unlinked, the player breaking the link chooses on which of the two possible squares the occupying piece remains. (It would have taken fewer words to say "Occupied squares may not be unlinked", but I like this rule better!)
Rule 2c2: "After all possible links have been made" seems to mean that every square is linked to another except the two squares on board 0 where the Kings are, and the two squares on board 1 that can have no partners because the King squares are unavailable.
However, it could also mean "after a certain number of links have been made" if you limit the number of active links.
The reason for limiting the number of active links is that the game becomes too confusing when there are too many links. My common sense tells me that the game is too confusing with unlimited links, and my chess variant design experience tells me that the tactical and strategic possibilities will be improved by having fewer links than squares.
What should the limit be?
Rule 2d: No more than 8 different pairs of squares may be linked at the same time. The number 8 is a guess. It may be changed if anybody who is interested enough to try playing Nick Danger Chess concludes that more or fewer are better.
Rule 3: "Linking a square" means that you name two squares, and those two squares become one.
Rule 3a: In the basic game, one (or both) of the linked squares must be empty; otherwise the multiple occupancy rules of All Go Together Chess should apply.
Rule 3b: In the basic game, the squares must be on different boards.
Rule 3c: In the basic game, both squares must be on the same rank; in games where this rule is not used, Pawns may promote only when making a Pawn move on board 0, from the 7th rank of board 0 to the 8th rank of board 0.
In Colorboundmost Nick Danger Chess, both squares must be the same color.
Note 3c1: When the boards are different sizes, the rule that both squares must be on the same rank cannot be used. Sometimes "both squares must be in the same quadrant" is a good rule.
Rule 3d: In the basic game, both squares must be unlinked; you cannot link to a square that is already linked, otherwise you could be in three or more places at once.
Rule 3e: You may link a square on board 0 to the same square on board 1.
If you require all links to be same-square links, the game is still interesting even on two boards, and therefore there is no reason to prohibit same-square links.
Suggestion 3e1: When you play with more than two boards, try using the rule that one of the two squares must be on board 0, (or in the case of infinite-board Nick Danger Chess, one of the two must be the lowest-numbered board that still has an unlinked square,) and that the other square must be the same square on your choice of other chessboards.
The previous paragraph assumes that infinite-board Nick Danger Chess allows infinite links, but because links can be broken and remade, this need not be so.
Suggestion 3e2: Playing with 4 boards, sized 8x8, 6x6, 4x4, and 2x2, it is logical to use the rule that links must be between adjacent boards.
Rule 3f: Of course, you may not link to the square containing the King.
Rule 4: There is no rule 4. I always wanted to say that.
There is no need for a fourth rule in the basic game.
Rule 4a: In Partial Information Infinite-Board Nick Danger Chess, of course you do not know what links your opponent has made, but in addition you should be able to see only boards on which you have pieces; also, you should be able to make one move on each board (but remember that each move to or from a linked square counts as a move on two different boards -- perhaps three boards if it's both to and from.)
When you link to a new board, the board gets the next number in sequence. Suppose that the highest-numbered board you know about is board 5, and you link to a new board, and the moderator tells you that the new board is number 8; now you have learned something about your opponent's links.
Rule 4b: Should Alice's Nick Danger Chess use two boards or four? This is a quandary which I cannot dequand. Perhaps both games are worthwhile.
Because all links are on the same rank, pieces with narrow moves may benefit more than pieces with wide moves, and pieces with Rookish moves benefit much less than pieces with diagonal or skewed moves.
Because Nick Danger Chess is not value-preserving, the well known armies that are equal on the normal chessboard may be mismatched in this game.