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Excellent Antoine! Thanks!

Does it enforce that only one Bishop can castle per side?

>> Is there any provision in Modern Chess allowing the Bishops to 
>> Change to opposite Diagonals?

Yes there is.

There is a 'Bishop-Castling' or 'Bishop Adjustment' rule.

For only ONE of the bishops, you can 'castle' (switch positions) with either the Knight, Queen or Minister next to it, therefore moving that Bishop to the white squares.

The Bishop in question must have not moved before 'castling', and the Knight, Queen or Minister to 'castle' with must have not moved either.

I'm trying to get a copy of the original book from Gabriel Maura from a friend in Puerto Rico, to formalize the rules, and request an update to the rules page in Chess Variants.

Haven't heard of the Bishop's one square ortogonal move in Modern Chess, but will ask for the clarification.

Sissa is a 9x9 chess variant invented in 1998 by Carlos Cetina, using his Bishops Conversion Rule. I do not recall anyone else including his rule in a chess variant. In Caïssa Britannia (Fergus Duniho) the Bishop may also step one space orthogonally without capturing - any Bishop, any number of times.

Just created a new preset for Modern Chess:
/play/pbm/play.php?game%3DModern+Chess%26settings%3DModern+Chess

I just discovered this variation in Puerto Rico while on vacation there when I played chess with some friends at a park in Manati.

The real purpose of this post is as follows: The Eight Officers Problem is impossible. Since the goal of Modern Chess was to increase mobility, that suggests that the 'Nine Officers Problem' should have a solution.

The Eight Officers Problem, for traditional chess, is the following: Is it possible to take a king, a queen, 2 bishops, 2 knights, and 2 rooks, and place them on a blank 8x8 board (with bishops on opposite colored squares) so that every square is attacked? (A piece does not attack the square it occupies.)

The Nine Officers Problem would be: Is it possible to take a king, a queen, a Prime Minister, 2 bishops, 2 knights, and 2 rooks, and place them on a blank 9x9 board (with bishops on Same-colored squares) so that every square is attacked?

Modern Chess was invented by Gabriel Maura in 1968. In 1972 a controlling body (FENDAM) was formed with delegates representing 16 countries. The first Modern Chess World Championship was held in Puerto Rico 1974. The World Correspondence Championship of Modern Chess was held in the years 1976-1983. Organized events seized in 1983.

Although a fully playable variant, it is curious why 'Modern Chess' attracted that much attention. If the Prime Ministers (Cardinals) are exchanged, what remains is a slow and drawish version of orthochess. The pawns cannot meet immediately in the middle of the board. The fact that the bishops move on the same colour depletes the strategical content, I suppose.
/Mats

It was suggessted on the Bishop Conversion Rule page that it could be used
with Modern Chess.

http://www.chessvariants.com/varvar.dir/bcr.html

I dug out my Modern Chess set because I remembered that this issue had
been addressed in the rules. According to the Mathematical Thesis of
Modern Chess, by Gabriel Vicente Maura, (2nd revised edition, 1974, page
34 note), once during a game a player may do 'the adjustment of the
Bishop'. This move allows the player to interchange the positions of
either Bishop and the adjacent Knight. Like castling, neither piece may
have moved, and the action counts as a move. This may be done only once
per game, and both players have to agree to allow this move before the
start of the game. This move is not allowed by the World Federation of
Modern Chess, but the author recommends familiarizing oneself with playing
with Bishops on different colors. This 50-page booklet also contains a
justification of the design of Modern Chess based on mobility
calculations, and two photographs of Mr. Maura with his chess set.