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This page is written by the game's inventor, Ralph Betza.

Inertia Chess (The Idea)

Joao Pedro Neto wrote to me about a chess variant that used the idea of inertia, and naturally I had to storm in with my own ideas.

The instant I read "Inertia", I had a mental image of pieces continuing to move after they were given their initial impetus; but when I read on, I found that yes the piece with inertia would continue to move, but no you couldn't move another piece (that is, only one piece would move per turn).

Momentum Chess (Inspiration)

Inertia is the wrong word. Momentum is the right word.

Even though I had a clear idea of how I thought the game should work, it took me a few tries before I came up with the right rules.

I put them on the web, Joao challenged me to a game, and after a few games I realized that Momentum Chess is a really great game, but that there were a few confusing things about the rules, and that some of the rules just added complexity to the rules without making the game itself any better.

Therefore, I have revised the rules just a bit. The email games that were played under the old rules are still readable, but not all of the moves are still the ones that would be chosen under the new rules.

It's a great game, but still a bit confusing. Because you can have more than one piece moving at once, you tend to forget that you can only move one piece at a time. For an example, see the second game.

Description of the Changes

The rule of check was more than confusing, it was confused.

For example, consider a position with a White Bishop on c4, a Black King on e8: after 1. Bc4-b5, Black is not in check because the White Bishop cannot play 2. Bb5-e8; however, if White has a King on a7, now it was thought to be check because the King can move to a6, which would stop the Bishop from moving and allows it to go to e8.

The problem is that after 2.Ka7-a6, White has already used his move, and 2.Bb5:e8 is not possible.

Under the corrected rules, 1.Bc4-b5 does not give check. Black cannot move to a6 because he would put himself in check. If White had a Pawn with momentum on a4, so that the move was "1.Bc4-b5,a4-a5" then of course this move would be check: next turn, the Pawn would advance to a6, and the Bishop would autostop and be free to go to e8.

This allows the Bishop to do two things in one move, but this is okay because it is permitted in two or three other situations. I thought it would make things easier to understand if I added a rule that says "no piece can do two things in one turn except in the following cases". That new rule is not really a change, and in a sense is not even a separate rule but merely a clarification of what the other rules have said.

In addition, I took out the restriction that "check must be covered first". This rarely seems to matter.

The Rules of Momentum Chess: Rules Only

  1. The rules of FIDE chess apply except as specifically modified by the following.

  2. When a piece is moved, it gains momentum, and continues to move in the same direction at the same speed on subsequent turns.

  3. When a piece with momentum cannot move any further, it simply stops. Now it has no momentum. We call this "autostop".

  4. On each turn, each player may either start one piece moving or use his move to stop the motion of one piece that was already in motion.

  5. On each turn, each player may have one or more pieces in motion by momentum, in addition to the usual move that may be made; the player has a free choice about which event should be resolved first.

  6. A piece can do two things in one turn in the following situations:
  • You can use your move to stop the motion of a piece after it has moved by momentum.

  • You can start a piece moving after it has autostopped.

  • Kings do not have momentum.
  • Momentum could force you to expose yourself to check.
  • It can be hard to see check. If you allow your King to be captured, you have lost the game.

  • The inventor of this game prefers that you play with Different Armies.

    The Rules of Momentum Chess: Rules and Clarifications

    1. The rules of FIDE chess apply except as follows.

    2. When a piece is moved, it acquires momentum, and keeps moving in the same direction at the same speed on subsequent turns.

      The "speed" of a piece is how many squares per turn it moves. After 1. e2-e4, the White Pawn will continue on with 2. e4-e6 unless it is stopped.

      Capture and non-capturing moves are different "speeds". Thus, after 1. Ng1-f3 e7-e5, White cannot use the inertia of the Knight to capture Nf3:e5.

    3. When a piece with momentum cannot move any further, it simply stops. Now it has no momentum. We call this "autostop".

    4. On each turn, each player may either start one piece moving or stop the motion of one piece that was already in motion.

      If a piece is moving by inertia, you cannot start it moving in another direction, nor can you change its speed. You can only stop it, which uses up your move.

    5. On each turn, each player may have one or more pieces in motion by momentum, in addition to the usual move that may be made. Doctor Einstein informs me that these events cannot all happen at once, because there is no true simultaneity.

      The player has a free choice about which event should be resolved first.

      For example, after 1. e4; 1...d6???; 2. e6, Bb5!; this move is check because White can (and will) play his third move as follows: first the Bishop is stopped because it can go no further, then the Bishop is chosen as the piece to be moved.

      As another example, consider a position with White Bishop f1, Black King g6, Black Pawn b5: 1. Bf1-d3+ can be answered by 1...b5-b4 (!). Now the Bishop cannot go to g6 because its momentum wants to carry it to b5, so it is not check.

      This is a most important rule, and deserves a bit more discussion.

      You can stop the motion of a piece either before or after it has moved by inertia, but of course if you do so you may not start some other piece moving.

      You can start a piece moving after you have allowed it to stop itself; a piece stops itself when it has no further moves in the same direction and speed.

      You can prevent a piece from stopping itself by getting out of its way; thus, if you have previously moved your Bishop from f1 to d3, and you have a Pawn on c4, and you plan to move that Pawn away from c4,

      it is your choice whether to allow the Bishop to stop itself on d3 before you move the Pawn,

      or to move the Pawn first and allow the Bishop to continue on to b5.

      On the other hand, if you have previously moved your Bishop from f1 to c3, and you have a Pawn on c3, and you plan to move the Pawn to c4, you get to choose whether the Bishop should stop at d3 or continue to b5, and the way you make this choice is by controlling the order in which things happen.

    6. A piece can do two things in one turn in no situations except the two mentioned above.

      If a Pawn's momentum carries it in to be promoted, you cannot move the new Queen until next turn because it still has the Pawn's momentum, and the autostop won't happen until next turn. The new Queen still has the Pawn's momentum because Rene Descarte tells me it's still the same object despite its altered form.

      Notice that a piece does not make a momentum move and an autostop both on the same turn.

    7. Kings do not have momentum.

      However, when you Castle, the Rook has momentum.

    8. Momentum could force you to expose yourself to check; after 1.e4; 1...d6??; 2.Bb5+!,e6; 2...c6,d5; 3.Qh5; Black is in check because the Pawn at c6 must continue on to c5, so the only way to avoid mate is 3...Qd7,c5,d4; which is obviously hopeless.

      Notice that 1.e4; 1...d6??; 2.Bb5+!,e6; 2...Nc6,d5; 3.Qh5?!; 3...d4,B:e6; is a bit better for Black because the Pawn at d4 stops the Knight on c6 from continuing on its way and exposing the King.

    9. It can be hard to see check. If you allow your King to be captured, you have lost the game.

    10. The inventor of this game prefers that you play with Different Armies.

      However, be warned that not all combinations of armies work with these rules.

      Philosophy of the Rules

      The major idea of the game is that you can use momentum to get many pieces in motion at the same time, and thereby create an irresistible attack.

      The minor idea of the game is that the pieces moving by their own momentum are out of control. Having too much of your army be out of control could cause you some difficulties.

      The major idea might be better expressed on a larger board, and with weaker pieces. On the 8x8 board, the pieces cannot move very far before they stop themselves, but in Tamerlane's Great Chess (for example), momentum could produce some spectacular battles.

      The rule about stealth checks is there to give the game some whimsy. Situations where this rule applies are likely to be rare and memorable.

      An Actual Game

      This game was played with the old rules. Black's 23rd move would not be check under the new rules, but under the old rules it was thought to be check because ...Kg8-f8 would autostop the Rook on d8 (but then the Rook couldn't go from d8 to d1 because Black had already used his move, so you see the old rules were wrong).

      If in doubt, read about the Remarkable Rookies and the Nutty Knights. 

      The first game of Momentum Chess ever played.

White: Joao Neto (Remarkable Rookies)
Black: Ralph Betza (Nutty Knights)

1. e4                   (fhNrlbK)c8-d6
2. e4-e6, f2-f3         (fhNfrlRK)d8:e6, (fhNrlbK)d6-e4
3. f3-f4, (HFD)f1-e2    (fhNrlbK)e4-e4, (fhNfrlRK)e6:f4
4. (HFD)e2-e2           (fhNfrlRK)f4-f4
5. (HFD)c1-c4           c7-c5
6. (HFD)c4-c4           (fhNfrlRK)f4:e2+
7. Ke1:e2               (fhNrlbK)e4-d4+
8. Ke2-d3               (fhNrlbK)d4:c4
9. Kd3:c4               (fhNrlbK)f8-e6
10. c2-c3               (fhNrlbK)e6-e6
11. Kd3, c3-c4          (fbNF)b8-c6
12. Ke2                 (fbNF)c6-d4,(fhNrlbK)e6-f4
13. Ke1                 (fbNF)d4-e2,(fbNF)g8-f6
14. (RN)d1-c3           (fbNF)e2-d3+,(fbNF)f6-e4
15. Ke1-d1,(RN)c3-b5    O-O
16. (WD)g1-e1           (fhNrlbK)f4:g2,(frlRbK)f8-d8
17. (WD)b1-b3,(WD)e1-c1 (fbNF)d3:c4,(frlRbK)d8-b8
18. (RN)b5:c5,(WD)b3-b5 (fbNF)c4:b5,d7-d5
19. (RN)c5-c5           (fbNF)e4-f3,d5-d3
20. (RN)c5:d3           (fbNF)b5-c4
21. (RN)d3:f3           (fbNF)c4-d3,(fhNrlbK)g2:f3
22. (R4)h1-f1           (fbNF)d3-e2+,(fhNrlbK)f3:d2
23. Kd1:d2,(R4)f1-d1    (fbNF)e2-f1,(frlRbK)b8-d8+
24. Ke1                 (fbNF)f1-g2,(frlRbK)d8-f8
25. (R4)d1-d3           (fbNF)g2-h3,f7-f5
26. (R4)d3-d3           f5-f3,Ra8-d8
      Black's first move was already an attempt to win a Pawn: if e4 stopped where it was, it could be taken, but if it went to e6, the fhNrlbK could continue to e4 while something else captured on e6.

      White's second move was very clever. Now if the fhNrlbK continued to e4, f3-f3 would attack it, and its momentum would prevent it from retreating. The clever part was that f2-f3 opens f2 and makes this possible.

      In fact, Black could gladly allow e6:f7+ (fbNF)g8:f7; but the temptation to sacrifice a piece for a pretty checkmate was too great. The idea was 3.f3-f3 (fbNF)g8-f6,(fhNrlbK)e4-f2+ 4. K:f2 (fbNF)f6-e4+,g7-g5 and mate next move.

      White was too smart for that, and soon the HFD at c4 attacked two pieces and Black must lose something. Instead of just losing a piece, Black gave up the Queen and got a position with almost even material and an exposed White King.

      Probably White had some better move in the part of the game from moves 10 to 13, but I don't know what. As it happened, Black advanced many small pieces with momentum, and got a position that looked extremely menacing.

      There may have been a way for Black to win the RN for two pieces, but I was trying for mate; in any case, my calculations at this stage of the game were full of mistakes. I was having trouble keeping track of the momentum.

      Black's attack came to nothing, and it became necessary to trade pieces and liquidate rather than to lose material. (fbNF)d3:c4 was the move for this.

      White made a serious mistake with 18. (RN)b5:c5,(WD)b3-b5 -- instead of this, he could have done much better by changing the order of resolution to first allow the Nb3 to autostop at b3 and then let the RN capture at c5.

      In the game, Black could not keep all the extra material because of momentum, but started a new mating attack with the help of the temporary extras. The threat of 21...(fbNF)f3-e2+,(fbNF)c4-d3 induced White to give up the Queen to save the King.

      22...(fbNF)d3-e2+,(fhNrlbK)f3:d2 forgot about the momentum of 24.(R4)f1-d1, but probably there was no way to avoid losing a piece. Even after losing a piece, Black has a huge material advantage.

      It's amusing how Black's "Rook" crosses the board again with its momentum. It gives check from d8 because of ...Kg8-f8, and the check allows Black to at least save the fbNF.

      In this game, Pawns can run rather quickly to make new Queens. White's resignation is justified.

      Second Sample Game

      White: Joao Neto   (Fabulous FIDEs)
Black: Ralph Betza (Forward FIDEs)

1.  e2-e3             (fhNbB)g8-f6
2.  e3-e4, Nb1-c3     e7-e5
3.  Ng1-f3, Nc3-d5    c7-c6
4.  Nd5-e7, Bf1-c4    (fBbhN)d8:e7,c6-c5
5.  Bc4-b3            O-O,c5-c4
6.  O-O               (fhNbB)f6:e4,c4-c3
7.  b2:c3             (fhNbB)e4-e4
8.  Rf1-e1            (fhNbB)e4-g6
9.  Nf3:e5            (fBbhN)e7-h4
10. Ne5-e5            (fhNbB)g6:e5
11. Re1:e5            (fsQbhNbK)d8-f6
12. Re5-e2            d7-d6
13. d4                (fBbhN)c8-g4,d6-d5
14. f3                (fhNbB)b8-d7
15. Bf4, f:g4         (fsQbhNbK)f6:f4,(fhNbB)d7-f6
16. g3!               (fsQbhNbK)f4:g4,(fhNbB)f6-h5
17. g3:h4             (fsQbhNbK)g4:h4,f7-f5
18. Re1?              f5-f3,Kg8-h8!
if 19. Rf1               f3-f2+
if 20. Kg2               (fsQbhNbK)h4-e4+
if 21. Kh3               (frlRrlbK)f8-f3+
if 22. Kg2               (frlRrlbK)f3-g4++
if 23. K:f2              (frlRrlbK)a8-f8+
      Both players were confused. Black could not play 4...Bf8-c5,c6:d5 and therefore White could have saved the N with 4. Nd5-d5.

      The same confusion occurred at White's 15th move. White made two moves, but neither of them was a continuation by momentum. This is why I decided to explain things more clearly in the new rules.

      The position after 16.g3! is quite interesting. White wins back a piece, and then Black has Q and N near the enemy King, but can't do anything right away because of momentum: Nh5-f4 wants to continue to d3, Nh5-g3 wants to continue to f1, (fsQbhNbK)h4-g5 isn't even check because it wants to continue to f6, and so on.

      Despite the two illegal-move errors, both players do a much better job of controlling their momentum.

      Third Sample Game

      White: Ralph Betza (Remarkable Rookies)
Black: Joao Neto   (Nutty Knights)

1.  (RN)d1-e3             (fbNF)g8-h6
2.  (RN)e3-f5,(HFD)f1-f4  (fhNrlbK)f8-e6
3.  (HFD)c1-c4            (fhNrlbK)e6-e6
(3...f7-f6??? 4.(HFD)f4-f7,(RN)f5:g7++ mate)
4.  (HFD)f4-e3            (fhNrlbK)c8-d6 ?
5.  (HFD)e3-e5+           (fhNrlbK)d6-e4,Ke8-f8
6.  (HFD)c4:e4            d7-d6
7.  (WD)g1-g3             d6-d6
8.  (HFD)e5-h5,(WD)g3-g5  (frlRbK)h8-g8
9.  h2-h4                 (fbNF)h6:g5
10. h4:g5                 g7-g6

I didn't see this coming at all. Normally, I wouldn't miss a simple
Pawn fork, but because it's Momentum Chess, I simply ruled out
thinking about Pawn moves because they usually don't attack
anything. Except, of course, when they have something in front of
them to stop their momentum!

11. (RN)f5-h6             (fhNrlbK)d6:g5 ?
12. (RN)h6:h7+            Kf8-e8
13. (RN)h7:g5             resign

} -I thought 11...g6:h5 12. (HFD)e4-f5+ Kf8-g7 was the
} -most difficult. Don't remember if I found anything for me.
} -13 {(RN):h7+,(HFD)g6} puts your K in danger, but no obvious followup,
} -13 {f4,(HFD)g6} gives check at f6 next, but no obvious followup,
} -and 13 g6 just hangs my RN. Maybe 13 {(R4):h5,(HFD)g6} because h7:g6
} -or f7:g6 seems to allow (RN):h7 mate.
      White's quick attack means that Momentum Chess can't really be played with these two armies. Even if there's no forced win, the existence of such a forcing opening is bad for the game.

      After Playing a Few Games

      There is an extra level of tactical complexity in this game because you have to control the momentum of your pieces in such a way that it works for you instead of against you.

      Look at the 16th to 18th moves of the second sample game for an example of momentum working against you.