When playing Chess4 with two or more players, the chess pieces are placed on the game board with all the Kings at the left of their Queens. The chess pieces must be placed on the first two rows of the outer edge of the game board.

The coordinates below are based on the following assuptions:

1. The board is laid out as a “plus sign”, with the “bottom of the plus” being toward/next to the viewer
2. As with traditional chess, White's pieces are placed on the two ranks closest to the viewer (M and N)
3. As with traditional chess, Black's pieces are placed on the two ranks furthest from the viewer (A and B)
4. There is considered to be a 3×3 cell area in each corner which is “off the board”. In other words, Black's King-side Rook is in square A4 because A1-A3 are “off the board”.

The colors always play in specific order: White plays first, then clockwise to Silver, followed by Black, and finally Gold.

Players can only move their Chess pieces on their turn. If a player is placed in check, when three or more players are playing, that player must wait until their designated turn before that player can respond to the threat. In the event a player is placed in check, and a player following moves into a position causing the player in check to be checkmated, the King is awarded to the last player (who actually checkmated the King). If Method 2 of play is being used, a player who has captured another player's King can only play those pieces on the turn of the player who has been checkmated and must continue to move those pieces on that player's turn only.

Pawns move forward only, unless attacking in a diagonal-forward manner. Pawns cannot attack diagonal in a backward manner. In the event a Pawn reaches King's row to the left, right, or directly across, that Pawn shall receive all the privileges of a Pawn reaching King's row during a traditional Chess game.

The traditional rules of Chess apply when playing Chess4. However, there are two methods in which Chess4 can be played:

1. Method 1 - Once a King has been checkmated, that player is out of the game, the remainder of the Chess pieces are left on the game board and play continues.
2. Method 2 - Once a King has been checkmated, the player who checkmated the King is now able to use the remaining Chess pieces of the captured King against the remaining opponents.

Players can choose partners for team play. Each partner can sit across from the other. For a more challenging game, players should not inform their partners as to what pieces to move or alert their partners when pieces are in danger. Two players can also play with two sets of Chess pieces each for a more challenging game.

1. A player is only eliminated by checkmate if he is checkmated at the beginning of the checkmating player's next turn.
2. An eliminated player's pieces are not considered “active” when assessing check/checkmate/stalemate.
3. An eliminated player's pieces cannot be “jumped over”, but they can be captured (and thus removed from the board), including the King.
4. It is an illegal move to capture an active player's King.
5. A player may not move his King into check (even if the checking piece will be forced to move before the player's next turn).
6. For purposes of determining which player gets “credit” for eliminating another player under Method 2, the following rules apply:
   1. Checkmate occurs at the beginning of the eliminated player's turn.
   2. If multiple players checkmate the King (two players' Knights, for example) then credit goes to the first player whose move resulted in a checkmate.
   3. If at any time the checkmated King becomes un-checkmated, the situation resets and credit goes to the first player who re-establishes the checkmate. For instance, if Gold moves his Queen to checkmate Black, and White captures Gold's Queen with his own Queen, then White has removed the checkmate (however momentarily) and reapplied it…so White would get credit for eliminating Black. However, if after White's move Silver moved his Queen to the same place (capturing White's Queen) then that would reset the checkmate a second time and ultimately give Silver credit for the elimination.
7. En passant
   1. En passant is also valid for perpendicular pawns. In other words, given a standard starting position, consider the following: “1. L11 … … K11”. This is a slightly different situation from standard chess, since it's still potentially possible for White's pawn to capture when/if Gold's pawn moves to K10. However, I'd say the en passant rule should still apply and White should be able to capture with “2. K12” (but not “2. K10”; you must always capture “behind” the captured pawn).
   2. If a player chooses to play en passant (regardless of whether the opponent's pawn is parallel or perpendicular), he must do so on his first opportunity. This is similar to the standard rule, with the following Chess4 clarification and extension:
      1. Other opponents' moves may occur between the move of the passing pawn and en passant, as long as the en passant is performed at the earliest opportunity. For example, if Silver passes a Pawn on White, White does not lose the opportunity to en passant simply because Black and Gold play their moves before White's next move.
      2. In the case of double en passant (for example, both Silver and Gold pass a pawn on White in the same round of turns), White may play en passant on both as long as he plays both moves on his next two turns. White chooses which passing pawn to capture first. The other passing pawn may not capture on this subsequent turn, but it may move; if it does, White may still capture (White's pawn's destination square remains unchanged).
      3. In the case of triple en passant, equivalent rules apply. White may capture all three passing pawns as long as he plays all three captures on his next consecutive three turns. The second and third passing pawns may move (but not capture) on their subsequent turns, but can still be captured (again, White's pawns' destination squares remain unchanged).

1. A player may castle a King acquired from an eliminated player if the eliminated player's King and castling Rook have not moved away from their starting positions since that player was eliminated. In other words, if Silver eliminates Black after Black has moved his King and/or his Rooks, Silver may still castle if he moves both the King and the castling Rook back to their original places. Neither piece may move “away” (either by rank or file) from its starting position (each move of that piece must bring it at least one rank or file closer to, and no ranks or files further from, its starting position). Once either piece is on its starting location, it may not move again (except to perform the castle) or castling is again illegal.
2. A player only retains control of one set of opponent's pieces. If a player eliminates a second player (and another player remains in the game) then that player must immediately make a permanent decision as to which eliminated opponent's pieces he will control. The other opponent's pieces remain on the board, but are static as with Method 1. The player may not choose to give up his own pieces to control both eliminated opponents'.
3. A player may only choose to control the piece of an opponent he directly eliminated. In other words, if Gold eliminates Silver, then Black eliminates Gold, Black may not choose to control Silver's pieces.
4. In cases of “discovered check” or “discovered checkmate”, credit for the move goes to the player who made the move that created the situation, even if the attacking piece belongs to an opponent. For instance, if Black creates a discovered checkmate by moving his Knight, which creates checkmate on Silver's King by revealing a discovered attack by Gold's Queen on Silver's King, then Black would get credit for eliminating Silver (even though Gold's Queen was actually performing the attack).
5. An eliminated player's King no longer has special significance for winning the game. If an opponent controls an eliminated player's King, he is not obligated to move it out of check (in fact, an eliminated player's King is never considered to be in check), nor will he incur any penalty if it's captured (other than the obvious loss of the piece).

Now that we've played through our first full Chess4 game, I'm putting the finishing touches on a Chess4-specific PGN sub-format. It won't be directly usable by current PGN parsers of course, but any human reading the file (who's familiar with Chess4 and PGN files) should immediately understand what's going on. My goals with the format are to maintain that familiarity while requiring the smallest adjustment possible if a piece of software ever did want to support the format. The example below just goes through the fourth turn, so I'm not constantly filling this wiki page's edit history up by adding moves; once the game finishes I'll update the whole thing at once and until then you can see the current PGN for the game in progress.

• A player is awarded one point for eliminating another player
• A player is awarded half a point for stalemating another player
• A player is awarded half a point for being stalemated
• When a player resigns, one point is divided evenly between the remaining players

This is just a quick lookup table for the “Result” header. I wanted to stay consistent with regular PGN (by using “1/2” intead of “0.5”), but it just got weird when I had to deal with something like “1-1/2” when the separator character between players is already ”-” (although, could do something like “1+1/2”; might need to anyway for the “incremental” scoring since it can produce thirds of a point). What may work is to double all the values so there are no halves…honestly, figuring out what the values of the numbers need to be for proper scoring ratios is probably a job for someone with a much more extensive chess background.

Don't forget when copy/pasting the Result header into your PGN; you need to rearrange the values to match the actual result of your game. In other words, the scores are always in the order of “White-Silver-Black-Gold” (just like in regular PGN where “1-0” always means that White won).

1. The “Result” header is in a different format
2. Colors always go in order: White, Silver, Black, Gold
3. Some extensions to algebraic notation are necessary:
   1. The '#' and '##' notations for check and checkmate are now followed by one or more of the letters “wsbg” to denote which color(s) has been checked or checkmated
   2. The 'o-o' and 'o-o-o' castling notations have been extended to include the color of pieces used (in case a player is castling another eliminated player's pieces)
   3. A new notation to specify an untaken turn (by an eliminated player)

Here are two example PGN files for completed games. They're actually both for the same game; the first one is written up with my (decidedly-amateur) annotations, and also contains all the extra “fluff” notation (using x to denote captures, annotating check/checkmate/elimination with #/##/###, etc.). This is the one you should look at if you want to actually follow the game, or be led gently when developing a parser. The second version is the same PGN with all comments and non-derivable information stripped; it's intended as an “acid test” for a c4pgn parser. Any feature-complete Chess4 engine should be able to take “firstgame-condensed.pgn” as input and produce “firstgame.pgn” as output (in other words, derive and populate the extra annotations from the remaining data). Well, without my Lord of the Rings-inspired color commentary, of course.

firstgame.pgn

[ Variant              "Chess4"                                               ]
[ Variant-Version      "1.0"                                                  ]
[ Notation-Style       "c4pgn"                                                ]
[ Notation-Version     "1.0"                                                  ]
[ White                "Inky"                                                 ]
[ Silver               "Blinky"                                               ]
[ Black                "Pinky"                                                ]
[ Gold                 "Clyde"                                                ]
[ Date                 "2013.09.18"                                           ]
[ Date-Started         "2013.09.18"                                           ]
[ Date-Ended           "2014.02.20"                                           ]
[ White-Eliminated-By  "Silver"                                               ]
[ Silver-Eliminated-By ""                                                     ]
[ Black-Eliminated-By  ""                                                     ]
[ Gold-Eliminated-By   "Silver"                                               ]
[ Result               "0-3-1-2"                                              ]
[ Result-Second-Format "0-6-2-4"                                              ]
 
1.  k7        { M7  > K7  } { White  }
    Nk3       { J1  > K3  } { Silver }
    d8        { B8  > D8  } { Black  }
    h11       { H13 > H11 } { Gold   }
 
2.  Bk9       { N6  > K9  }
    Nd3       { E1  > D3  }
    Nc6       { A5  > C6  }
    Nf12      { E14 > F12 }
 
3.  k10       { M10 > K10 }
    e4        { E2  > E4  }
    c4        { B4  > C4  }
    Qxk10     { G14 > K10 }
 
4.  l9        { M9  > L9  }
    Nf4       { D3  > F4  }
    d4        { C4  > D4  }
    Qxk9      { K10 > K9  }
 
5.  l8        { M8  > L8  }
    Ne6       { F4  > E6  }
    Bb8       { A9  > B8  }
    Qj10      { K9  > J10 }
 
6.  Nl11      { N10 > L11 }
    Be2       { F1  > E2  }
    d10       { B10 > D10 }
    Qe10      { J10 > E10 }
 
7.  Nl6       { N5  > L6  }
    O-O       { G1  > E1 / D1  > F1 }
    d7        { B7  > D7  }
    { [A(Clyde)
    For Black's own sake, I hope he's not planning on
    ( 8. ... ... Bxi14 ), because that leaves him open to ( 9. ... Bxb5 ),
    which leads to the loss of a pawn and knight
    ( 9. ... Bxb5 Ra5 ... 10. ... Bxc6 ) followed by the loss of the ability
    to castle ( 10. ... ... Ka9 ) or a bishop trade
    ( 10. ... ... Bb7 ... 11. ... Bxb7 Qxb7 ).  Although, Silver might avoid
    the bishop trade since there are more than two players to think about.
    ] }
    Qf10      { E10 > F10 }
 
8.  Rn10      { N11 > N10 }
    Ng5       { E6  > G5  }
    Bxi14?    { A6  > I14 }
    Kxi14??   { H14 > I14 }
    { [A(Clyde)
    I wish I could say I had some grand strategy here, but no; I just wasn't
    paying attention. :(
    ] }
 
9.  Rxf10     { N10 > F10 }
    Bxl9??    { E2  > L9  }
    Bf4       { B8  > F4  }
    Nh13      { J14 > H13 }
 
10. Bm8       { N9  > M8  }
    Be2       { L9  > E2  }
    Qa6       { A7  > A6  }
    g12       { G13 > G12 }
 
11. Nj7       { L6  > J7  }
    Nh3       { G5  > H3  }
    Bb8       { F4  > B8  }
    Nd11      { F12 > D11 }
 
12. Rxf13     { F10 > F13 }
    Bxj7      { E2  > J7  }
    Qxg12     { A6  > G12 }
    Nf12      { D11 > F12 }
    { [A(Clyde)
    Well, that sucked.  Have to move back to avoid checkmate by protecting
    H13...
    ] }
 
13. Rxh13     { F13 > H13 }
    Be2       { J7  > E2  }
    O-O-O     { A8  > A6 / A4  > A7  }
    Bg13      { F14 > G13 }
 
14. Rxi13     { H13 > I13 }
    Bm10      { E2  > M10 }
    d11       { B11 > D11 }
    Kj14      { I14 > J14 }
 
15. Nk9       { L11 > K9  }
    Be2       { M10 > E2  }
    e11       { D11 > E11 }
    j12       { J13 > J12 }
 
16. Ri8       { I13 > I8  }
    j3        { J2  > J3  }
    xf12      { E11 > F12 }
    Bxf12     { G13 > F12 }
 
17. Ri14??    { I8  > I14 }
    Bj2       { I1  > J2  }
    Rxh11     { A11 > H11 }
    Rxi14     { D14 > I14 }
 
18. O-O       { N7  > N5 / N4  > N6  }
    Bg5       { J2  > G5  }
    Rxm11     { H11 > M11 }
    Kj13      { J14 > J13 }
 
19. Ni10      { K9  > I10 }
    Bxm11     { G5  > M11 }
    Nc11      { A10 > C11 }
    d12       { D13 > D12 }
 
20. Nh8       { I10 > H8  }
    Bh6       { M11 > H6  }
    Nd13      { C11 > D13 }
    Bi9       { F12 > I9  }
 
21. Nf7       { H8  > F7  }
    Bj4       { H6  > J4  }
    Ra11      { A7  > A11 }
    Bj10      { I9  > J10 }
 
22. k5        { M5  > K5  }
    Bi3       { J4  > I3  }
    Rj11      { A11 > J11 }
    Rd14      { I14 > D14 }
 
23. l4??      { M4  > L4  }
    { [A(Clyde)
    Well, that's...interesting.  I pretty much based my entire strategy for the
    previous move on the assumption that White would, you know, take the free
    rook that Black served up on a platter.
    ] }
    Ni4       { K3  > I4  }
    Qg11#g    { G12 > G11 }
    Ki13
 
24. Bxj11     { M8  > J11 }
    j4        { J3  > J4  }
    e8        { D8  > E8  }
    Bh12      { J10 > H12 }
 
25. Nh8       { F7  > H8  }
    xk5       { J4  > K5  }
    Qe12      { G10 > E12 }
    Bg11      { H12 > G11 }
 
26. Bm8       { J11 > M8  }
    Rj1       { K1  > J1  }
    Bc7       { B8  > C7  }
    Rkg14     { K14 > G14 }
 
27. Bxi4      { M8  > I4  }
    k6        { K5  > K6  }
    Qxe13#g   { E12 > E13 }
    Kh14      { I13 > I14 }
    { [A(Clyde)
    Really grasping at straws here...I'd like to think about Bi13 at some
    point, to protect myself from White's Queen; I thought about
    ( 27. ... ... ... Ki14 ) but I need the extra protection so I can do
    Rg13 to threaten White's Queen and still protect the piece...otherwise
    Black will get a lot more than a free Bishop.
    ] }
 
28. Bxk6??    { I4  > K6  }
    Bxn8      { I3  > N8  }
    Bxg11     { C7  > G11 }
    Rde14     { D14 > E14 }
 
29. Rxn8      { N6  > N8  }
    Rj4       { J1  > J4  }
    Bf12#g    { G11 > F12 }
    Ki14      { H14 > I14 }
 
30. Bf11#bg   { K6  > F11 }
    Rj5#w     { J4  > J5  }
    e10       { D10 > E10 }
    Kj14      { I14 > J14 }
 
31. Kn6       { N5  > N6  }
    Qi1       { H1  > I1  }
    Bg11#g    { F12 > G11 }
    Rxg11     { G14 > G11 }
 
32. Bk6       { F11 > K6  }
    Rxb5      { J5  > B5  }
    Qxe14#g   { E13 > E14 }
    Ki13      { J14 > I13 }
 
33. Rn9       { N8  > N9  }
    Qxl4#w    { I1  > L4  }
    Na7       { C6  > A7  }
    Re11      { G11 > E11 }
 
34. Kn7       { N6  > N7  }
    Rk5#b?!   { B5  > K5  }
    { [A(Clyde::?!)
    Interesting that this wasn't ( 34. ... Ri5#bg ), although now that I think
    about it, it was probably deliberate; by not putting me in check, he
    enabled me to take Black's Queen (which is good for him).  Still, this
    breathed new life into what was pretty much looking like an impending
    checkmate.
 
    Hold your ground! Hold your ground!
    Sons of Gold, my brothers,
    I see in your eyes the same fear that would take the heart of me.
    A day may come when the courage of Gold fails,
    when we forsake our Pawns
    and break all bonds of piece protection,
    but it is not this day.
    An hour of silver Rooks and shattered pawn structure,
    when the age of Gold comes crashing down,
    but it is not this day!
    This day we fight!!
    By all that you hold dear on this good Earth,
    I bid you stand, men of the Gold!!!
    ] }
    Nb5       { A7  > B5  }
    Rxe14     { E11 > E14 }
 
35. Bxd13     { K6  > D13 }
    Rxb5      { K5  > B5  }
    Kb7       { A6  > B7  }
    Rj14      { E14 > J14 }
 
36. Rxb9#b    { N9  > B9  }
    Rn5#w     { B5  > N5  }
    Kc8       { B7  > C8  }
    k11       { K13 > K11 }
 
37. Km7       { N7  > M7  }
    Qxh8      { L4  > H8  }
    Kxb9      { C8  > B9  }
    d11       { D12 > D11 }
 
38. Bm4       { D13 > M4  }
    Rn9#b     { N5  > N9  }
    Kc8       { B9  > C8  }
    k10       { K11 > K10 }
 
39. Bk6       { M4  > K6  }
    Qk5#w     { H8  > K5  }
    c6        { B6  > C6  }
    j11       { J12 > J11 }
 
40. Kl7       { M7  > L7  }
    Rj1       { F1  > J1  }
    f10       { E10 > F10 }
    j10       { J11 > J10 }
 
41. l6        { M6  > L6  }
    Qn5#w     { K5  > N5  }
    g10       { F10 > G10 }
    j9        { J10 > J9  }
 
42. Km8       { L7  > M8  }
    Qn7##w    { N5  > N7  }
    h10       { G10 > H10 }
    Rk14      { J14 > K14 }
 
43. %s        { White checkmated by silver and forced to pass }
    Ba6#b###w { E2  > A6  }
    Kc7       { C8  > C7  }
    j7        { J8  > J7  }
 
44. %%%
    Qn5       { N7  > N5  }
    Kd6       { C7  > D6  }
    j6        { J7  > J6  }
 
45. %%%
    Qi10#g    { N5  > I10 }
    Ke5       { D6  > E5  }
    Kh12      { I13 > H12 }
 
46. %%%
    Rj4       { J1  > J4  }
    d6        { C6  > D6  }
    k9        { K10 > K9  }
 
47. %%%
    Rh4       { J4  > H4  }
    e7        { D7  > E7  }
    xl8       { K9  > L8  }
 
48. %%%
    Rxh10#g   { H4  > H10 }
    Kxe4      { E5  > E4  }
    Kg11      { H12 > G11 }
 
49. %%%
    Rn11#g    { N9  > N11 }
    f7        { E7  > F7  }
    Kf12      { G11 > F12 }
 
50. %%%
    Qi12#g    { I10 > I12 }
    f8        { E8  > F8  }
    Kg13      { F12 > G13 }
 
51. %%%
    Rh13      { H10 > H13 }
    g8        { F8  > G8  }
    Kg14      { G13 > G14 }
 
52. %%%
    Rxd11     { N11 > D11 }
    Ke5       { E4  > E5  }
    Rh14      { J14 > H14 }
 
53. %%%
    Qe12#bg   { I12 > E12 }
    e6        { D6  > E6  }
    Kf14      { G14 > F14 }
 
54. %%%
    Rd14##g   { D11 > D14 }
    e4        { D4  > E4  }
    %s        { Gold checkmated by Silver and forced to pass }
 
55. %%%
    Kd1###g   { Gold eliminated by Silver }
    %%        { Black resigns }
 
    0-3-1-2   { Result: "Last Man Standing" scoring }
    0-3-0-0   { Result: Incremental scoring }

firstgame-condensed.pgn

[ Variant "Chess4" ]
[ Variant-Version "1.0" ]
[ Notation-Style "c4pgn" ]
[ Notation-Version "1.0" ]
[ White "Inky" ]
[ Silver "Blinky" ]
[ Black "Pinky" ]
[ Gold "Clyde" ]
[ Date "2013.09.18" ]
[ Date-Started "2013.09.18" ]
[ Date-Ended "2014.02.20" ]
[ Result "0-3-1-2" ]
 
1. k7 Nk3 d8 h11 2. Bk9 Nd3 Nc6 Nf12 3. k10 e4 c4 Qk10 4. l9 Nf4 d4 Qk9 5. l8 Ne6 Bb8 Qj10 6. Nl11 Be2 d10 Qe10 7. Nl6 O-O d7 Qf10 8. Rn10 Ng5 Bi14 Ki14 9. Rf10 Bl9 Bf4 Nh13 10. Bm8 Be2 Qa6 g12 11. Nj7 Nh3 Bb8 Nd11 12. Rf13 Bj7 Qg12 Nf12 13. Rh13 Be2 O-O-O Bg13 14. Ri13 Bm10 d11 Kj14 15. Nk9 Be2 e11 j12 16. Ri8 j3 f12 Bf12 17. Ri14 Bj2 Rh11 Ri14 18. O-O Bg5 Rm11 Kj13 19. Ni10 Bm11 Nc11 d12 20. Nh8 Bh6 Nd13 Bi9 21. Nf7 Bj4 Ra11 Bj10 22. k5 Bi3 Rj11 Rd14 23. l4 Ni4 Qg11 Ki13 24. Bj11 j4 e8 Bh12 25. Nh8 k5 Qe12 Bg11 26. Bm8 Rj1 Bc7 Rkg14 27. Bi4 k6 Qe13 Kh14 28. Bk6 Bn8 Bg11 Rde14 29. Rn8 Rj4 Bf12 Ki14 30. Bf11g Rj5 e10 Kj14 31. Kn6 Qi1 Bg11 Rg11 32. Bk6 Rb5 Qe14 Ki13 33. Rn9 Ql4 Na7 Re11 34. Kn7 Rk5 Nb5 Re14 35. Bd13 Rb5 Kb7 Rj14 36. Rb9 Rn5 Kc8 k11 37. Km7 Qh8 Kb9 d11 38. Bm4 Rn9 Kc8 k10 39. Bk6 Qk5 c6 j11 40. Kl7 Rj1 f10 j10 41. l6 Qn5 g10 j9 42. Km8 Qn7 h10 Rk14 43. % Ba6 Kc7 j7 44. %%% Qn5 Kd6 j6 45. %%% Qi10 Ke5 Kh12 46. %%% Rj4 d6 k9 47. %%% Rh4 e7 l8 48. %%% Rh10 Ke4 Kg11 49. %%% Rn11 f7 Kf12 50. %%% Qi12 f8 Kg13 51. %%% Rh13 g8 Kg14 52. %%% Rd11 Ke5 Rh14 53. %%% Qe12g e6 Kf14 54. %%% Rd14 e4 % 55. %%% Kd1 %% 0-3-1-2 0-3-0-0