Whatever Abbott may have done to package the ideas of Fairy Chess into Baroque chess, it is clear that Kuttner and Moore had prior art.
In 1963, at the suggestion of his publisher, Robert Abbott changed the name of Baroque chess to Ultima, by which name it is now commonly known. Abbott considers his own "invention" to be flawed, and has suggested several amendments to the rules, but these suggestions have been substantially ignored by the gaming community, which continues to play by the 1963 rules.
Baroque chess can be played with the standard chessboard and pieces, although this can be confusing at first to players familiar with the normal game. All the pieces except for the king have different methods of capture, and all but the pawn have different names. The initial setup of the pieces is the same as in standard chess, except that the white king and queen exchange squares, and the rooks on a1 and h8 are inverted.
The names of the pieces and rules for movement are as follows:
The white king moves c4-d5 delivering checkmate. Normally it would not be possible for the two kings to be adjacent, but here the black king is unable to move due to the white immobilizer on e4, thus the d5 square is not under attack by black, and the white king is not moving into check.
Note that white could not play c4-d4, as that would place his own king in check from the black withdrawer. Capturing the withdrawer with c4-d3 would result in stalemate, as black would then have no legal moves.
The white pawn moves g4-d4, capturing the black immobilizer and black pawn. The black withdrawer on e5 is not captured, because pawns capture only vertically and horizontally, not diagonally. The black chameleon on d3 is not captured, because there is no white piece on d2. Finally, the black long-leaper on g3 was safe because it moved between the two white pawns, rather than a white pawn moving to complete the custodial capture.
The white withdrawer moves g6-d3, capturing the black pawn on h7. The pawn on g7 and the chameleon on h6 are unaffected because the withdrawer did not move in their respective lines, but the withdrawer could have captured either by a move in the sixth rank or g-file respectively. Note that the withdrawer also gives check to the black king by threatening to move away on the d-file.
The white long-leaper moves d2-d4-d6-d8, capturing three black pieces. It might instead have captured the black withdrawer with either d2-g5 or d2-h6. On the other hand, the black pawn on b2 and the black chameleon on d1 are safe from the long leaper because there is no square on the opposite side on which the long leaper could land. Also the black pawns on f2 and g2 cannot be captured by d2-h2, because there is no space in between the two pawns which would allow the long leaper to make two separate jumps. A move of d2-b4 would be illegal because long leapers may not jump over friendly pieces.
The white coordinator moves d4-f6, capturing black's long-leaper on c6 and immobilizer on f2. If white had played d4-d6 instead, he would have captured black's long-leaper and pawn. The coordinator threatens only pieces on the same rank or file as the friendly king.
The white immobilizer moves f3-d5, immobilizing 5 black pieces. The black long-leaper on g4, which had been immobilized, is now free to move again.
An immobilizer can never be captured by a king, withdrawer, immobilizer, or chameleon. However, when an immobilizer comes into contact with an enemy chameleon or immobilizer, the two pieces freeze each other, after which neither can move the rest of the game unless the other is captured.
An immobilized piece is allowed to commit suicide, i.e. be removed from the board, in lieu of the regular move of that player. There may be strategic reasons to open a line. For example, after the above diagrammed move, black may wish to commit suicide with his long-leaper on c5, so that his other long-leaper will be able to capture the white immobilizer by jumping over it on the fifth rank. White cannot hinder this plan, because the immobilizer is itself immobilized by the black chameleon.
In the above diagram, the white chameleon moves g6-e6-c6, astoundingly capturing all seven black pieces in one move and delivering check.