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OOmost Chess

By Ralph Betza

The basic idea of Castlingmost Chess is that all moves should be Castling moves. The inspiration for the game was a joke made in response to Colorboundmost Chess.

The following sections treat the basic concepts of Castlingmost, followed by the specific rules of two forms of Castlingmost Chess.

Those who like the game may call it by its nickname, which is OOmost Chess (pronounced Oh-oh-most, sounds a bit like Kokomo).

Conceptual Castling

Castling is a move that involves two pieces which simultaneously move towards each other and land on or over the midpoint (exactly where they land is defined in the Geometry of Castling.

Rules of Castling

In Castlingmost Chess, of course Rule Zero applies, but several changes are made to the rules of Castling:

  1. Any piece (or Pawn or King) may Castle with any other. This includes Castling with enemies, so in the opening position it is legal to play 1. a2=d5+f7-c4, which means that the Pa2 Castles with the Pf7.

    Of course, you cannot make a move that consists of Castling two enemy pieces with each other, and when you Castle with an enemy piece, your piece is considered the primary -- the primary piece moves further when there an odd number of squares in between.

  2. Because all moves are Castling moves, pieces that have already moved are not forbidden to Castle.

  3. Pieces that are not Kings may Castle even if they are attacked or if they cross attacked squares. Kings are still limited by the usual rules, no Castling into, out of, or across attacked squares -- and this applies even if your King is not the primary.

    Please notice that because all moves are Castling moves, but a King may not Castle out of check, a check may easily be checkmate; however, one may Castle with something else to interpose a block, or one may Castle with the piece that's giving check to remove it from its attacking square.

    What if you Castle with the enemy King? For example, 1. Ke1-d2+d2-e1, d7-d4+Kd2-d5 check. I believe this must be allowed because it makes the game more interesting; and after all, consider the purpose of forbidding the K to castle in cases involving check -- surely these rules were never meant to apply to cases where you can move the enemy King!

  4. If a Pawn reaches its eight rank by Castling, the Pawn is promoted, and the player who makes the move chooses its new rank (it may be promoted to any type of piece other than K or P that was in the game at the start of the game).

  5. A miscellaneous rule: if you are stalemated, you lose.

  6. Another miscellaneous rule: it is forbidden to repeat a previous position.

The Geometry of Castling

When there are an even number of squares between the two pieces, both land just over the midline -- as in O-O. For example, if the WP on a2 Castles with the BP on b2, they simply exchange places.

In fact, this example is forbidden because it is a null move. Instead, a WP on a2 may Castle with a BP on b2, trading places; and in response, Castling a2 with b2 is of course forbidden because it is an "undo" of the move that just happened.

The even number of squares may be 0, 2, 4, or 6; and

  • castling a1 with a2, distance 0, the pieces simply trade places.

  • castling a1 with a4, distance 2, a1 goes to a3 and a4 goes to a2.

  • castling a1 with a6, distance 4, a1-a4 and a6-a3.

  • castling a1 with a8, distance 8, a1-a5 and a8-a4.

If there are an odd number of squares in between, the piece that initiated the castling manoeuvre goes to the midpoint and the other piece jumps over it -- as in O-O-O.

The piece that initiated the Castling is the "primary".

  • castling a1 with a3, distance 1, a1-a2 and a3-a1

  • castling a1 with a5, distance 3, a1-a3 and a5-a2

  • castling a1 with a7, distance 5, a1-a5 and a7-a4.

Why Two Forms?

Castling is always a movement and never a capture. If all moves are Castling, how do you win?

Two answers are presented here, but others are also possible.

Castlingmost Chess I -- Pictorial Check

The original form of OOmost Chess has no captures at all; instead, the game is won by pictorial check and thus pictorial checkmate.

For example, after 1. Ke1-e2+e2-e1, Qd8-e7+e7-d8 gives Check. The Q cannot really capture the King, but it looks like check, so it is check!

The idea of this game is that all the power is always on the board, which should cause a tense situation. In fact, since Pawns can be promoted, and since promotion is the only way to gain a material advantage, the armies get stronger as the game goes on.

Not only do the armies get stronger, but the board also shrinks! If a1 becomes empty, no piece can ever move there again; and after a1 is empty, emptying b1 creates another dead square! Thus, as the game goes on, more power is concentrated in less space, until at last it explodes.

I haven't examined the game much. I think that games will be long except when blunders happen -- but it will be very easy to blunder.

OOmost I Sample

1. Ke1-e2+e2-e1, Qd8-e7+e7-d8 gives Check. The reply is to Castle with a Pawn in such a way that you block the line from e7 to e2. (Castling the Pe1 with the Ke2. even though the P initiates the Castling move, is still "castling out of check", an illegal move.)

1. Ke1-e2+e2-e1, Qd8-e7+e7-d8 check 2. h2-e5+c7-f4 Qe7-e6+e5-e7 gives check again, but is a risky response because it brings the WP to its 7th rank. In reply, 3. a2-c4+Qe6-b3 is forced but 3...Qb3-c4+c4-b3 is check again and b3-c4 is an illegal "undo". 4. c2-d3+Qc4-c2, Qc2-d2+d2-c2 and you can see that because it is forbidden to repeat a previous position W is mated in a couple of moves.

It is possible that a move such as 1. Qd1-e2+e2-d1 forces a win. I haven't analyzed it.

Castlingmost Chess II -- Normal Capture

In OOmost II, all non-capturing moves are by Castling, but all captures are completely normal.

The strategic idea of this game is that movement becomes ever more difficult as pieces are captured; and if you are stalemated, you lose.

I haven't examined this game at all. I think it should be both playable and enjoyable.

Different Armies

Consider a piece that moves as a Chancellor but captures as Ferz (mRmNcF in my funny notation). Its ideal value would be the average of its two components, roughly a Rook.

Compare it with a piece that moves a Ferz but captures as Chancellor, mFcRcN. Its ideal value would be the same as mRmNcF, and perhaps their practical values would also be equal -- the mFcRcN has enormous capturing power, but its minimal movement makes it difficult to aim it at anything; and the mRmNcF can hope to use its great mobility to amplify its tiny capture power by forking things.

Whhether these pieces are actually worth a Rook, or more, or less, is one of those questions of piece values that I have not investigated. Maybe someday somebody will.

The point is, in OOmost Chess, all movements are the same, and pieces differ only in their ability to capture (or to give pictorial check). The mFcRcN would be enormously more powerful than the cFmRmN in OOmost Chess.

Armies whose pieces have capture powers same as movement powers should be able to have an even game of OOmost Chess, and most existing armies fill the bill.


It will probably be fun to play OOmost Chess I or II a time or two, but I doubt that either will become a popular and widely-played game.

This is one of those cases where the idea of the game is so interesting that it really doesn't matter if it ever gets played.

Written by Ralph Betza.
WWW page created: December 5th, 2002.