Multiple Occupancy Miscellany
Recently, I have been investigating various possible forms of rules
that would allow multiple pieces on the same square.
This article concludes the series by presenting a few odds and ends,
scraps left over from the mill.
Rules about Leaving
Most of the Crowd Chess games have assumed that any piece which is
on a square where there are multiple pieces may freely move off the
"Don't Go" would be the name for a game where a crowd must always be
multicolored, so that the last piece of one color would be unable to
leave the crowded square unless the opponent also had only one piece
In this game, capturing into a crowd would remove one enemy piece
from the crowd; because a crowd must always be multicolored, the
last enemy piece in a crowd cannot be captured!
An unexpected effect of this rule can be shown in an example: if
the square e4 contains two White Pawns and a Black Queen, the Queen
cannot move -- is immobilized by the two Pawns -- although adding
another Black piece to the crowd would free the Queen, in a
change of hostages. However,
adding another Black piece would also make the Q vulnerable to capture!
I don't think the "Don't Go" rule makes a good game, at least not by
itself. Perhaps in combination with something else?
Stacking and Queueing
Pritchard mentions a game called Stacking Chess; although
the full rules are not given, it is known that multiple pieces of
one color could be on the same square, that all would be captured at
once, and that a special additional rule allowed Castling onto
occupied squares. What is not mentioned is the rule about leaving
Unless we assume that the name of the game was poorly chosen, this
game must have implemented the well-known "last in, first out"
protocol of stacking. For example, if White played Ng1-f3 and then
played f2-f3, the Pawn could move but the Knight could not; or, if
f2-f3 were played first and then Ng1-f3, the Knight could move but
the Pawn could not. The obvious logical complement to stacking would
Limited occupancy would be another type of rule. If the fire
department has determined that occupancy of a single square by more
than 4 pieces is dangerous and unlawful, the limit is by number;
limited occupancy by piece type would permit no more than one Pawn
(or King or Rook, etc.) to be on the same square; limited occupancy
by weight would permit (for example) no more value on a square than
represented by one Rook, or five Pawns, and so on.
The Reverse of a Crowd
Instead of having multiple pieces on one square, how about one piece
on many squares? It's
What happens when you make a capturing move that ends on a square
where there is more than one enemy piece? Many rules are possible.
The simple cases are that you capture
However, there are many other possibilities. I'll mention only one.
one enemy piece, chosen by the
attacker (in this case multicolored crowds are common),
all enemy pieces, as in
pieces, friend or foe,
pieces at all.
In many wargames, multiple units on a square are called a "stack",
and when the stack is attacked one unit takes care of the defense.
In a chess variant, one might have a rule that capturing into a
crowd removes one enemy piece from the board, that piece to be
chosen by the defender.
First, the simple case of voluntary crowd formation:
If crowds are dangerous, the rules should allow crowds to be formed
freely, simply by moving onto a square that already contains either
a crowd or a friendly piece. If crowds are advantageous, the rules
should make crowd formation more restrictive (as in
Safety in Numbers).
One of the other possibilities is involuntary crowd formation, for
Falling Off, the momentum of a careening piece can put it on the
same square as another friendly piece. There are endless
possibilities for involuntary crowds, as they could be formed by
momentum, by teleportation, by parachute (for example imagine a
variant of Chessgi in which you are
forced to place reserves on friendly-occupied squares, and crowd
capture grabs all enemy pieces; the character of the game is
different -- less intense and explosive perhaps -- and it's a new
Another way to limit crowd formation is to permit like pieces to
start a crowd, after which any friendly piece can join.
Once there were laws against loitering (such laws violated the right of
peaceful assembly, and are laws no more). A cop would tell a crowd
to break up and keep moving along the sidewalk; and so in any crowd
more than one turn old, every piece but one would move to an
adjacent square. Pawns could reach the first rank, or the 8th
without promotion, and new crowds could be formed this way to break up
next turn (because pieces that are dispersing can freely move onto
occupied squares -- and cannot capture or promote, by the way).
First you make a legal move (get out of check!) and then disperse
crowds that were formed by your last move (you choose what goes
where), including crowds that were formed by your last move's
dispersal phase. Crowds are formed freely, captures take one enemy
piece off the board. All dispersals take place at the same time, in
effect, so the order doesn't matter, and no piece moves more than
once in a single dispersal phase -- but a piece can move into an
existing crowd and then be dispersed. Since crowds can be recreated
by other dispersals, the same square might have a crowd every single
Loitering Chess is not play tested. There's some chance that it's an
incredibly good game. Both players make as many crowds as possible
because it gives extra mobility; and things get complicated. Maybe
When I started to think about having multiple pieces on the same
square, it was in the context of a type of Subway Chess (not the
same as the game that already used that name!). When I learned that
there already existed some chess variants with multiple pieces on a
square, I was at first disappointed; but then I realized that their
use of the concept of the crowd was limited and unimaginative, and I
But then I realized that it was a great opportunity! Crowd Chess was
rather unexplored, so I could dash in and plant my flag on all the
notable landmarks; and I did. Perhaps I've overlooked a few major
mountain ranges or inland seas, but I've certainly made a try at
discovering all the easily-discovered ideas in Crowd Chess.
 You may consider this as a challenge. There's gold in them thar hills!
Written by Ralph Betza.
WWW page created: April 23, 2001.