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It isn't possible to force checkmate with King vs. King, Knight & Knight. A checkmate position, however, is still possible to achieve, so does that mean the 50 move rule applies, or does insufficient material mean insufficient material to force checkmate?

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    Checkmate is not possible unless the side with the King also has a pawn, so the pawn may move and prevent stalemate.
    – Tony Ennis
    Commented May 29, 2012 at 12:17
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    Checkmate is possible if the lone K side plays incorrectly, however unlikely they are to play the losing move.
    – PeskyGnat
    Commented May 29, 2012 at 17:04
  • @TonyEnnis true but only if the pawn is on certain squares
    – yobamamama
    Commented Dec 31, 2019 at 0:46

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This is a complicated situation, and it is made even more complicated by the fact that USCF and FIDE rules differ slightly.

First and most importantly, the 50 move rule always applies. If 50 moves are made without a capture or a pawn move, then the game is simply declared drawn.

The rules about insufficient material only are important when one side runs out of time. Basically, the rule says that if a player runs out of time, and the opponent does not have enough material to mate, then the game is declared drawn instead of the side with time remaining getting the win. For example, if black has a lone king and white flags while trying to mate with a king and queen, the game is a draw. If there is insufficient material to deliver mate, an OTB game can also be declared drawn even with time on the clock if a tournament director is summoned. This will only apply for cases like K vs K or KN vs K. Usually it is not an issue because the opponent will simply agree to a draw.

So now to the specific case of KNN vs K:

FIDE

Under FIDE rules for standard tournament time controls, if the side with just a king runs out of time, then the side with KNN wins because a mating position can be constructed. If the side with just the king avoids one move mates, however, then mate cannot be forced.

USCF

Under USCF rules, if the side with just a king runs out of time, then the game is declared drawn. The USCF focuses on the fact that checkmate cannot be forced.

Online Play

Many online chess servers will automatically stop a game when there is not enough material left on the board to mate. In the specific case of KNN vs K most servers do not automatically give a draw (following the FIDE rules). One interesting case is KN vs KB when Playchess and ICC will stop the game and award a draw (not following FIDE rules) while Chesscube follows the FIDE rules and continues play.

Practical Play

Now for practical play, if you are defending with just the king, it is very easy to avoid mate for 50 moves, even if you have almost no time remaining. Make sure that you don't move to a square where mate can be delivered next move and you will never be forced into mate.

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    Note that the 50 move rule (FIDE rules) doesn't always apply, it must be claimed. From july 2014 onwards there will be an additional 75 moves rule that is automatic, in case neither side wants to claim for some reason.
    – RemcoGerlich
    Commented Mar 15, 2014 at 18:39
  • +1 for the fact that the 50-move rule is not automatic, it must be claimed. Same goes for the 3-move repetition (and there comes a 5-move repetition alternative that is now automatic). A comment on the USCF rule: in a huge percentage of positions with several pieces on board, a checkmate cannot be forced, although possible. That rule claiming that it simply cannot be forced, therefore it's a draw, is too simplistic in my opinion. If a player runs out of time, there must be more severe consequences, for example that side must have shown in the final moves that he's not gonna be checkmated.
    – downhand
    Commented Apr 17, 2015 at 8:39
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I only know FIDE rules.

K+N+N v K is a position in which checkmate is still possible, therefore it is treated exactly like any other position in which checkmate is still possible (like, say, the opening position).

In particular, this means that:

  • This position is not declared a draw, like dead positions are (K+B v K, for instance, is an immediate draw not requiring any claims)

  • If the side with only a King runs out of time, he loses

But there are a few other rules to mention besides the 50 move rule:

  • If it's rapid or standard chess, there is no increment, and play is in the last time period (in which there's therefore only a limited amount of time for the rest of the game), if the defending side has less than 2 minutes on the clock, he can claim a draw under Rule 10.2 claiming that the position can't be won any more in normal fashion. Rule 10.2 is always complex but in this case it's obvious that the draw claim will hold.

  • There is a rule against "bringing the game of chess into disrepute" and I feel that an arbiter should give a warning based on that rule to any player that knows better continuing this in a slow game.

But in Blitz, you are out of luck and need to press your clock faster than he does.

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K+N+N vs. K is a draw or stalemate. The knights cannot force checkmate though helpmate is possible.

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    The game wouldn't be declared a draw by stalemate, stalemate can only ever happen when the players reach a position of stalemate. But you are correct that stalemate instead of checkmate is more likely.
    – Andrew ♦
    Commented May 29, 2012 at 12:50
  • I fixed my wording. But the game will never end by checkmate.
    – Tony Ennis
    Commented May 29, 2012 at 22:32
  • edit - I was unaware that the lone K could helpmate himself, but he can. I did not know that.
    – Tony Ennis
    Commented May 29, 2012 at 22:46
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Any minimally courteous opponent will accept your offer of a draw in this situation.

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    For what it's worth, I would play on with the knights for 50 moves and hope that the weaker side blundered into mate. While it is trivial to make moves and not get mated, it's still very reasonable to test your opponent, same as you can try to win a drawn king and pawn vs lone king endgame.
    – Andrew ♦
    Commented Aug 16, 2014 at 5:45
  • @Andrew: Would a player wishing to avoid mate need any strategy beyond "If there is only one legal move, make it; otherwise, make any move other than Ka1, Ka8, Kh1, or Kh8."? I would consider it unsportsmanlike to press on in such a situation unless a win on time was a realistic possibility.
    – supercat
    Commented Oct 10, 2014 at 19:48
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Situations where mate is available but obviously cannot be forced are uncommon. E.g. KBvKB where bishops are opposite colours.

Under USCF rules, the game would most likely be declared a draw if time ran out for either side.

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